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	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,181,845
Available-for-sale Securities, Due after one year through five years, Amortized Cost	2,477,266
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	814,996
Available-for-sale Securities, Due after ten years, Amortized Cost	1,022,280
Available-for-sale Securities, Amortized Cost Basis	5,496,387
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,182,729
Available-for-sale Securities, Due after one year through five years, Fair Value	2,478,433
Available-for-sale Securities, Due after five years through ten years, Fair Value	812,786
Available-for-sale Securities, Due after ten years, Fair Value	997,966
Available-for-sale, at fair value	$ 5,471,914
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	2.10%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	2.27%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	2.90%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.14%
Available-for-sale Securities, Total Weighted-Average Yield	2.30%