Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2018 USD ($) position	Dec. 31, 2017 USD ($) position
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,211,860	$ 1,034,964
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	353,693	388,534
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,449)	(3,909)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ (2,093)	$ (2,106)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position	107	91
Farmer Mac Guaranteed Securities and USDA Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,102,932	$ 1,329,332
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,832,524	2,414,093
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11,973)	(8,169)
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(97,056)	(58,302)
Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,064,053	1,304,160
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	814,058	351,664
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11,558)	(8,094)
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(10,942)	(3,335)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,367,859	1,273,965
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,559,584	1,759,377
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(31,550)	(8,819)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ (37,315)	$ (37,492)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position	54	36
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | position	17	16
Held-to-maturity Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]
Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position	52	23
USDA Guarantees [Member] | Farmer Mac Guaranteed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 24,721
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
USDA Guarantees [Member] | USDA Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 38,879	451
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,018,466	2,062,429
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(86,114)	(54,967)
Available-for-sale Securities [Member] | USDA Guarantees [Member] | Farmer Mac Guaranteed Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(73)
Available-for-sale Securities [Member] | USDA Guarantees [Member] | USDA Securities [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ (415)	$ (2)