Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Held-to-maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Farmer Mac Guaranteed Securities and USDA Securities [Member]
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 4,211,181	$ 4,244,124
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Held-to-maturity Securities, Fair Value	$ 4,102,728	4,179,772
Held-to-Maturity Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Weighted Average Yield	3.36%
Held-to-Maturity Securities, Debt Maturities, After Ten Years, Weighted-Average Yield	3.54%
Farmer Mac Guaranteed Securities [Member]
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	$ 750,013
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	1,359,953
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	218,630
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,067,307	2,126,274
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	745,474
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	1,340,138
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	209,806
Held-to-maturity Securities, Fair Value	$ 2,045,382	$ 2,116,891
Held-to-Maturity Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Held-to-Maturity Securities, Debt Maturities, Next Twelve Months, Weighted-Average Yield	2.10%
Held-to-Maturity Securities, Debt Maturities, Year Two Through Five, Weighted-Average Yield	2.99%
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	3.09%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,882,585
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	$ 1,807,310