Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Held-to-maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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	$ 4,244,124	$ 3,158,456
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Held-to-maturity Securities, Fair Value	$ 4,179,772	3,066,644
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	3.09%
Held-to-Maturity Securities, Debt Maturities, After Ten Years, Weighted-Average Yield	3.36%
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	$ 866,294
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	1,293,467
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	202,137
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,126,274	1,149,231
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	864,819
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	1,284,120
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	197,048
Held-to-maturity Securities, Fair Value	$ 2,116,891	$ 1,153,009
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	1.73%
Held-to-Maturity Securities, Debt Maturities, Year Two Through Five, Weighted-Average Yield	2.29%
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	2.68%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,882,226
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	$ 1,833,785