Loans and Allowance for Credit Losses (Troubled Debt Restructurings by Class) (Detail) - JPY (¥)
12 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Mar. 31, 2020
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment ¥ 265,569,000,000 ¥ 178,416,000,000 ¥ 217,911,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 263,450,000,000 177,246,000,000 216,599,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 38,521,000,000 39,450,000,000 26,521,000,000
Commercial | Loans acquired with deteriorated credit quality      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 0 0 10,786,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 0 0 10,786,000,000
Commercial | Domestic      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 168,348,000,000 39,282,000,000 61,735,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 167,342,000,000 39,282,000,000 61,735,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 12,020,000,000 16,179,000,000 8,857,000,000
Commercial | Foreign      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 16,144,000,000 33,839,000,000 39,827,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 16,144,000,000 33,839,000,000 39,827,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 13,180,000,000 9,861,000,000 2,337,000,000
Residential      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 22,484,000,000 18,121,000,000 5,137,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 22,484,000,000 18,121,000,000 5,137,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 203,000,000 157,000,000 31,000,000
Card      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 20,937,000,000 20,857,000,000 22,625,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 19,824,000,000 19,737,000,000 21,561,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 2,746,000,000 2,733,000,000 3,320,000,000
MUAH      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 11,384,000,000 22,801,000,000 33,782,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 11,384,000,000 22,763,000,000 33,564,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 0 3,437,000,000 4,656,000,000
Krungsri      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 14,869,000,000 18,548,000,000 31,238,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 14,869,000,000 18,548,000,000 31,209,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment 5,657,000,000 6,226,000,000 7,305,000,000
Other      
Financing Receivable, Modifications [Line Items]      
Troubled debt restructurings, Pre-modification outstanding recorded investment 11,403,000,000 24,968,000,000 12,781,000,000
Troubled debt restructurings, Post-modification outstanding recorded investment 11,403,000,000 24,956,000,000 12,780,000,000
Troubled debt restructurings that subsequently defaulted, Recorded Investment ¥ 4,715,000,000 ¥ 857,000,000 ¥ 15,000,000