Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Available-for-sale Securities (Details) - Farmer Mac Guaranteed Securities and USDA Securities [Member] $ in Thousands	Mar. 31, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,285,582
Available-for-sale Securities, Due after one year through five years, Amortized Cost	2,421,507
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	815,799
Available-for-sale Securities, Due after ten years, Amortized Cost	1,338,678
Available-for-sale Securities, Amortized Cost Basis	5,861,566
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,285,526
Available-for-sale Securities, Due after one year through five years, Fair Value	2,419,021
Available-for-sale Securities, Due after five years through ten years, Fair Value	800,742
Available-for-sale Securities, Due after ten years, Fair Value	1,334,098
Available-for-sale, at fair value	$ 5,839,387
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	2.45%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	2.56%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	3.03%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.84%
Available-for-sale Securities, Total Weighted-Average Yield	2.66%