Fair Value Disclosures - Unobservable Input Reconciliation (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   $ (235) $ 0 $ (235) $ (691)
Purchases       0 0
Sales       0 0
Settlements       0 0
Realized and Unrealized Gains/(Losses) included in Income       235 456
Unrealized Gains/(Losses) included in Other Comprehen-sive Income       0 0
Transfers Out       0  
Ending Balance $ 0     0 (235)
Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   6,770,031 5,472,078 6,770,031 6,421,447
Purchases     1,055,501 1,434,461 1,635,627
Sales     0 (26,675) 0
Settlements     (394,307) (1,153,191) (1,129,054)
Realized and Unrealized Gains/(Losses) included in Income     4,210 15,292 (19,049)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     (22,143) 64,946 (138,940)
Transfers Out       (1,632,786)  
Ending Balance 5,472,078   6,115,339 5,472,078 6,770,031
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance         (691)
Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   66,418 41,265 66,418 64,406
Purchases     0 0 233
Sales     0 (26,675) 0
Settlements     (657) (890) (798)
Realized and Unrealized Gains/(Losses) included in Income     359 (379) 455
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     4,448 2,791 2,122
Transfers Out       0  
Ending Balance 41,265   45,415 41,265 66,418
USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]          
Trading Securities, Unrealized Gains     900 1,800 500
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   1,612,013 1,771,532 1,612,013 1,590,783
Purchases     363,621 335,359 361,894
Sales     0 0 0
Settlements     (245,481) (228,585) (278,605)
Realized and Unrealized Gains/(Losses) included in Income     761 1,151 (1,274)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     26,886 51,594 (60,785)
Transfers Out       0  
Ending Balance 1,771,532   1,917,319 1,771,532 1,612,013
Available-for-sale Securities [Member] | Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   65,490 40,576 65,490 63,159
Purchases     0 0 233
Sales     0 (26,675) 0
Settlements     0 (205) (24)
Realized and Unrealized Gains/(Losses) included in Income     (100) (825) 0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     4,448 2,791 2,122
Transfers Out       0  
Ending Balance 40,576   44,924 40,576 65,490
Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   5,091,600 3,659,281 5,091,600 4,766,258
Purchases     691,880 1,099,102 1,273,500
Sales     0 0 0
Settlements     (148,169) (923,716) (849,651)
Realized and Unrealized Gains/(Losses) included in Income     3,090 14,520 (18,230)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     (53,477) 10,561 (80,277)
Transfers Out       (1,632,786)  
Ending Balance 3,659,281   4,152,605 3,659,281 5,091,600
Available-for-sale Securities [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   1,553,669 1,731,222 1,553,669 1,486,595
Purchases     363,621 335,359 361,894
Sales     0 0 0
Settlements     (233,385) (209,400) (234,035)
Realized and Unrealized Gains/(Losses) included in Income     0 0 0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     26,886 51,594 (60,785)
Transfers Out       0  
Ending Balance 1,731,222   1,888,344 1,731,222 1,553,669
Trading Securities [Member] | Investment Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   928 689 928 1,247
Purchases     0 0 0
Sales     0 0 0
Settlements     (657) (685) (774)
Realized and Unrealized Gains/(Losses) included in Income     459 446 455
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     0 0 0
Transfers Out       0  
Ending Balance 689   491 689 928
Trading Securities [Member] | USDA Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   58,344 40,310 58,344 104,188
Purchases     0 0 0
Sales     0 0 0
Settlements     (12,096) (19,185) (44,570)
Realized and Unrealized Gains/(Losses) included in Income     761 1,151 (1,274)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     0 0 0
Transfers Out       0  
Ending Balance 40,310   28,975 40,310 58,344
Institutional Credit [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   5,070,366 3,631,662 [1] 5,070,366 4,739,577
Purchases     678,566 [1] 1,091,475 1,273,500
Sales     0 [1] 0 0
Settlements     (138,687) [1] (922,908) (844,437)
Realized and Unrealized Gains/(Losses) included in Income     3,090 [1] 14,520 (18,230)
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     (53,387) [1] 10,995 (80,044)
Transfers Out 0 (1,600,000) 0 (1,632,786) 0
Ending Balance 3,631,662 [1]   4,121,244 [1] 3,631,662 [1] 5,070,366
USDA Guarantees [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   21,234 27,619 21,234 26,681
Purchases     13,314 7,627 0
Sales     0 0 0
Settlements     (9,482) (808) (5,214)
Realized and Unrealized Gains/(Losses) included in Income     0 0 0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     (90) (434) (233)
Transfers Out       0  
Ending Balance 27,619   31,361 27,619 21,234
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Floating rate auction-rate certificates backed by Government guaranteed student loans | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   65,285 40,576 65,285 63,159
Purchases     0 0 0
Sales     0 (26,675) 0
Settlements     0 0 0
Realized and Unrealized Gains/(Losses) included in Income     (100) (825) 0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income     4,448 2,791 2,126
Transfers Out       0  
Ending Balance 40,576   44,924 40,576 65,285
Floating Interest Rate [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance   205 0 205 0
Purchases       0 233
Sales       0 0
Settlements       (205) (24)
Realized and Unrealized Gains/(Losses) included in Income       0 0
Unrealized Gains/(Losses) included in Other Comprehen-sive Income       0 (4)
Transfers Out       0  
Ending Balance 0     0 205
Floating Interest Rate [Member] | Trading Securities [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]          
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]          
Beginning Balance [2]   $ 928 689 928 1,247
Purchases [2]     0 0 0
Sales [2]     0 0 0
Settlements [2]     (657) (685) (774)
Realized and Unrealized Gains/(Losses) included in Income [2]     459 446 455
Unrealized Gains/(Losses) included in Other Comprehen-sive Income [2]     0 0 0
Transfers Out [2]       0  
Ending Balance [2] $ 689   $ 491 $ 689 $ 928
[1] Includes unrealized gains of $0.9 million attributable to assets still held as of December 31, 2015 that are recorded in "Gains/(losses) on trading securities."
[2] Unrealized gains are attributable to assets still held as of December 31, 2015 and are recorded in "Gains/(losses) on trading securities."