Fair Value (Reconciliation of Assets and Liabilities Measured at Fair Value on Recurring Basis Using Level 3 Inputs) (Detail) - JPY (¥)
¥ in Millions
12 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains (losses) for the period included in earnings, Liabilities ¥ 8,315  
Fair Value, Measurements, Recurring    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 1,135,409 ¥ 881,017
Total gains (losses) for the period included in earnings, Assets 68,815 51,944
Total gains (losses) for the period included in other comprehensive income, Assets 32,308 (16,012)
Purchases, Assets 708,270 675,013
Issues, Assets 674 653
Sales, Assets (204,970) (191,734)
Settlements, Assets (570,658) (414,592)
Transfers into Level 3, Assets 136,272 188,587
Transfers out of Level 3, Assets (85,037) (38,161)
Level 3 assets, Ending balance 1,219,735 1,135,409
Change in unrealized gains (losses) included in earnings for assets still held at period end 105,473 29,112
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 liabilities, Beginning balance (2,212) 6,606
Total gains (losses) for the period included in earnings, Liabilities   (32,946)
Total gains (losses) for the period included in other comprehensive income, Liabilities (3,080) 937
Purchases, Liabilities 0 0
Issues, Liabilities 36,535 5,336
Sales, Liabilities 0 0
Settlements, Liabilities (19,361) (27,823)
Transfers into Level 3, Liabilities 5,364  
Transfers out of Level 3, Liabilities 1,372 (10,654)
Level 3 liabilities, Ending balance 16,463 (2,212)
Change in unrealized gains (losses) included in earnings for liabilities still held at period end 15,532 (17,043)
Fair Value, Measurements, Recurring | Discontinued Operations, Held-for-sale    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Total gains (losses) for the period included in earnings, Assets (2,589)  
Total gains (losses) for the period included in other comprehensive income, Assets 4,234  
Purchases, Assets 770  
Issues, Assets 0  
Sales, Assets 0  
Settlements, Assets (4,247)  
Transfers into Level 3, Assets 78,750  
Transfers out of Level 3, Assets 0  
Level 3 assets, Ending balance 76,918  
Change in unrealized gains (losses) included in earnings for assets still held at period end 1,461  
Fair Value, Measurements, Recurring | Disposal group, disposed of by sale, not discontinued operations    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 0  
Level 3 assets, Ending balance   0
Fair Value, Measurements, Recurring | Other    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 liabilities, Beginning balance (2,212) 6,606
Total gains (losses) for the period included in earnings, Liabilities 8,315 (32,946)
Total gains (losses) for the period included in other comprehensive income, Liabilities (3,080) 937
Purchases, Liabilities 0 0
Issues, Liabilities 36,535 5,336
Sales, Liabilities 0 0
Settlements, Liabilities (19,361) (27,823)
Transfers into Level 3, Liabilities 5,364 (7,686)
Transfers out of Level 3, Liabilities 1,372 (10,654)
Level 3 liabilities, Ending balance 16,463 (2,212)
Change in unrealized gains (losses) included in earnings for liabilities still held at period end 15,532 (17,043)
Fair Value, Measurements, Recurring | Trading account, Trading securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 756,413 525,946
Total gains (losses) for the period included in earnings, Assets 41,919 21,730
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 458,395 352,917
Issues, Assets 0 0
Sales, Assets (197,245) (189,980)
Settlements, Assets (262,150) (117,399)
Transfers into Level 3, Assets 665 190,391
Transfers out of Level 3, Assets 0 (27,192)
Level 3 assets, Ending balance 797,997 756,413
Change in unrealized gains (losses) included in earnings for assets still held at period end 40,113 23,477
Fair Value, Measurements, Recurring | Trading account, Debt securities | Foreign government and official institution bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 1,280 1,052
Total gains (losses) for the period included in earnings, Assets 287 625
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 31,591 49,637
Issues, Assets 0 0
Sales, Assets (31,432) (49,927)
Settlements, Assets (15) (59)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 (48)
Level 3 assets, Ending balance 1,711 1,280
Change in unrealized gains (losses) included in earnings for assets still held at period end 47 159
Fair Value, Measurements, Recurring | Trading account, Debt securities | Corporate bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 77 144
Total gains (losses) for the period included in earnings, Assets (68) 31
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 126 82
Issues, Assets 0 0
Sales, Assets (100) (180)
Settlements, Assets 0 0
Transfers into Level 3, Assets 648 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 683 77
Change in unrealized gains (losses) included in earnings for assets still held at period end (69) (5)
Fair Value, Measurements, Recurring | Trading account, Debt securities | Residential mortgage-backed securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 1,000 0
Total gains (losses) for the period included in earnings, Assets (2) 0
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 97,197 51,533
Issues, Assets 0 0
Sales, Assets (98,176) (50,533)
Settlements, Assets (19) 0
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 0 1,000
Change in unrealized gains (losses) included in earnings for assets still held at period end 0 0
Fair Value, Measurements, Recurring | Trading account, Debt securities | Asset-backed securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 336,811 416,259
Total gains (losses) for the period included in earnings, Assets 26,692 12,808
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 270,974 106,308
Issues, Assets 0 0
Sales, Assets (67,500) (83,700)
Settlements, Assets (251,746) (114,864)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 315,231 336,811
Change in unrealized gains (losses) included in earnings for assets still held at period end 27,693 16,309
Fair Value, Measurements, Recurring | Trading account, Debt securities | Other debt securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 277,635 6,651
Total gains (losses) for the period included in earnings, Assets 14,179 11,096
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 21,352 69,499
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets 0 0
Transfers into Level 3, Assets 0 190,389
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 313,166 277,635
Change in unrealized gains (losses) included in earnings for assets still held at period end 14,179 11,096
Fair Value, Measurements, Recurring | Trading account, equity securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 139,610 101,840
Total gains (losses) for the period included in earnings, Assets 831 (2,830)
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 37,155 75,858
Issues, Assets 0 0
Sales, Assets (37) (5,640)
Settlements, Assets (10,370) (2,476)
Transfers into Level 3, Assets 17 2
Transfers out of Level 3, Assets 0 (27,144)
Level 3 assets, Ending balance 167,206 139,610
Change in unrealized gains (losses) included in earnings for assets still held at period end (1,737) (4,082)
Fair Value, Measurements, Recurring | Trading derivatives—net    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 25,027 3,477
Total gains (losses) for the period included in earnings, Assets 23,754 45,361
Total gains (losses) for the period included in other comprehensive income, Assets 1,882 (104)
Purchases, Assets 51 597
Issues, Assets 674 653
Sales, Assets 0 0
Settlements, Assets (16,312) (20,689)
Transfers into Level 3, Assets 41,076 (2,159)
Transfers out of Level 3, Assets (3,715) (803)
Level 3 assets, Ending balance 71,089 25,027
Change in unrealized gains (losses) included in earnings for assets still held at period end 46,149 31,184
Fair Value, Measurements, Recurring | Trading derivatives—net | Interest rate contracts    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 14,179 (750)
Total gains (losses) for the period included in earnings, Assets (25,977) 14,401
Total gains (losses) for the period included in other comprehensive income, Assets 824 (32)
Purchases, Assets 0 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets 9,696 1,405
Transfers into Level 3, Assets 39,609 (99)
Transfers out of Level 3, Assets (838) (746)
Level 3 assets, Ending balance 37,493 14,179
Change in unrealized gains (losses) included in earnings for assets still held at period end 4,410 11,807
Fair Value, Measurements, Recurring | Trading derivatives—net | Foreign exchange contracts    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 7,283 5,005
Total gains (losses) for the period included in earnings, Assets 2,403 3,743
Total gains (losses) for the period included in other comprehensive income, Assets 206 (128)
Purchases, Assets 21 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets 1,833 140
Transfers into Level 3, Assets 1,467 (1,480)
Transfers out of Level 3, Assets (5,750) 3
Level 3 assets, Ending balance 7,463 7,283
Change in unrealized gains (losses) included in earnings for assets still held at period end (1,640) 3,812
Fair Value, Measurements, Recurring | Trading derivatives—net | Equity contracts    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 623 (4,222)
Total gains (losses) for the period included in earnings, Assets 46,579 27,299
Total gains (losses) for the period included in other comprehensive income, Assets 858 55
Purchases, Assets 30 3
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (27,543) (21,944)
Transfers into Level 3, Assets 0 (580)
Transfers out of Level 3, Assets 2,873 12
Level 3 assets, Ending balance 23,420 623
Change in unrealized gains (losses) included in earnings for assets still held at period end 42,546 15,660
Fair Value, Measurements, Recurring | Trading derivatives—net | Commodity contracts    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance (63) (65)
Total gains (losses) for the period included in earnings, Assets 30 1
Total gains (losses) for the period included in other comprehensive income, Assets (6) 1
Purchases, Assets 0 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (6) 0
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance (45) (63)
Change in unrealized gains (losses) included in earnings for assets still held at period end 30 1
Fair Value, Measurements, Recurring | Trading derivatives—net | Credit derivatives    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 2,271 2,727
Total gains (losses) for the period included in earnings, Assets 750 (166)
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 0 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (292) (290)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 2,729 2,271
Change in unrealized gains (losses) included in earnings for assets still held at period end 819 (182)
Fair Value, Measurements, Recurring | Trading derivatives—net | Other    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 734 782
Total gains (losses) for the period included in earnings, Assets (31) 83
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 0 594
Issues, Assets 674 653
Sales, Assets 0 0
Settlements, Assets 0 0
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 (72)
Level 3 assets, Ending balance 29 734
Change in unrealized gains (losses) included in earnings for assets still held at period end (16) 86
Fair Value, Measurements, Recurring | Available-for-sale debt securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 289,616 274,930
Total gains (losses) for the period included in earnings, Assets 277 6,710
Total gains (losses) for the period included in other comprehensive income, Assets 25,143 (14,559)
Purchases, Assets 236,786 308,672
Issues, Assets 0 0
Sales, Assets 0 (11)
Settlements, Assets (287,747) (276,234)
Transfers into Level 3, Assets 15,651 70
Transfers out of Level 3, Assets (66,790) (9,962)
Level 3 assets, Ending balance 212,936 289,616
Change in unrealized gains (losses) included in earnings for assets still held at period end 12,545 (2,779)
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Discontinued Operations, Held-for-sale    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 0  
Total gains (losses) for the period included in earnings, Assets (963)  
Total gains (losses) for the period included in other comprehensive income, Assets 3,791  
Purchases, Assets 1  
Issues, Assets 0  
Sales, Assets 0  
Settlements, Assets (4,247)  
Transfers into Level 3, Assets 66,680  
Transfers out of Level 3, Assets 0  
Level 3 assets, Ending balance 65,262 0
Change in unrealized gains (losses) included in earnings for assets still held at period end 5,737  
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Foreign government and official institution bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 16,718 15,767
Total gains (losses) for the period included in earnings, Assets 0 0
Total gains (losses) for the period included in other comprehensive income, Assets 607 (888)
Purchases, Assets 5,163 3,518
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (5,964) (1,679)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets (4,634) 0
Level 3 assets, Ending balance 11,890 16,718
Change in unrealized gains (losses) included in earnings for assets still held at period end 87 473
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Corporate bonds    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 162 10,108
Total gains (losses) for the period included in earnings, Assets (328) (8)
Total gains (losses) for the period included in other comprehensive income, Assets 68 (1)
Purchases, Assets 557 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (114) (45)
Transfers into Level 3, Assets 2,854 70
Transfers out of Level 3, Assets (110) (9,962)
Level 3 assets, Ending balance 3,089 162
Change in unrealized gains (losses) included in earnings for assets still held at period end (265) (10)
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Residential mortgage-backed securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 15 15
Total gains (losses) for the period included in earnings, Assets 0 0
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 0 0
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets 0 0
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 15 15
Change in unrealized gains (losses) included in earnings for assets still held at period end 0 0
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Commercial mortgage-backed securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 1,599 1,977
Total gains (losses) for the period included in earnings, Assets 0 0
Total gains (losses) for the period included in other comprehensive income, Assets 104 (346)
Purchases, Assets 0 0
Issues, Assets 0 0
Sales, Assets 0 (11)
Settlements, Assets (524) (21)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets (1,179) 0
Level 3 assets, Ending balance 0 1,599
Change in unrealized gains (losses) included in earnings for assets still held at period end 0 (163)
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Asset-backed securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 136,920 140,875
Total gains (losses) for the period included in earnings, Assets 580 6,718
Total gains (losses) for the period included in other comprehensive income, Assets 14,066 (7,958)
Purchases, Assets 231,039 267,767
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (258,226) (270,482)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets 0 0
Level 3 assets, Ending balance 124,379 136,920
Change in unrealized gains (losses) included in earnings for assets still held at period end 6,651 (4,047)
Fair Value, Measurements, Recurring | Available-for-sale debt securities | Other debt securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 134,202 106,188
Total gains (losses) for the period included in earnings, Assets 25 0
Total gains (losses) for the period included in other comprehensive income, Assets 10,298 (5,366)
Purchases, Assets 27 37,387
Issues, Assets 0 0
Sales, Assets 0 0
Settlements, Assets (22,919) (4,007)
Transfers into Level 3, Assets 12,797 0
Transfers out of Level 3, Assets (60,867) 0
Level 3 assets, Ending balance 73,563 134,202
Change in unrealized gains (losses) included in earnings for assets still held at period end 6,072 968
Fair Value, Measurements, Recurring | Investment securities, equity securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 45,569 39,963
Total gains (losses) for the period included in earnings, Assets 5,856 (3,231)
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 9,899 10,481
Issues, Assets 0 0
Sales, Assets (3,113) (1,725)
Settlements, Assets 0 0
Transfers into Level 3, Assets 130 285
Transfers out of Level 3, Assets (2,458) (204)
Level 3 assets, Ending balance 55,883 45,569
Change in unrealized gains (losses) included in earnings for assets still held at period end 3,960 (4,150)
Fair Value, Measurements, Recurring | Investment securities, equity securities | Nonmarketable equity securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 45,569 39,963
Total gains (losses) for the period included in earnings, Assets 5,856 (3,231)
Total gains (losses) for the period included in other comprehensive income, Assets 0 0
Purchases, Assets 9,899 10,481
Issues, Assets 0 0
Sales, Assets (3,113) (1,725)
Settlements, Assets 0 0
Transfers into Level 3, Assets 130 285
Transfers out of Level 3, Assets (2,458) (204)
Level 3 assets, Ending balance 55,883 45,569
Change in unrealized gains (losses) included in earnings for assets still held at period end 3,960 (4,150)
Fair Value, Measurements, Recurring | Other    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 18,784 36,701
Total gains (losses) for the period included in earnings, Assets (402) (18,626)
Total gains (losses) for the period included in other comprehensive income, Assets 1,049 (1,349)
Purchases, Assets 2,369 2,346
Issues, Assets 0 0
Sales, Assets (4,612) (18)
Settlements, Assets (202) (270)
Transfers into Level 3, Assets 0 0
Transfers out of Level 3, Assets (12,074) 0
Level 3 assets, Ending balance 4,912 18,784
Change in unrealized gains (losses) included in earnings for assets still held at period end 1,245 (18,620)
Fair Value, Measurements, Recurring | Other | Discontinued Operations, Held-for-sale    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Level 3 assets, Beginning balance 0  
Total gains (losses) for the period included in earnings, Assets (1,626)  
Total gains (losses) for the period included in other comprehensive income, Assets 443  
Purchases, Assets 769  
Issues, Assets 0  
Sales, Assets 0  
Settlements, Assets 0  
Transfers into Level 3, Assets 12,070  
Transfers out of Level 3, Assets 0  
Level 3 assets, Ending balance 11,656 ¥ 0
Change in unrealized gains (losses) included in earnings for assets still held at period end ¥ (4,276)