Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Schedule of Held-to-maturity Securities (Details) - USD ($) $ in Thousands	Jun. 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	$ 4,205,710	$ 4,244,124
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Held-to-maturity Securities, Fair Value	$ 4,108,772	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	3.30%
Held-to-Maturity Securities, Debt Maturities, After Ten Years, Weighted-Average Yield	3.50%
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	$ 769,783
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	1,368,924
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	212,947
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,093,092	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	766,311
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	1,351,656
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	205,159
Held-to-maturity Securities, Fair Value	$ 2,074,587	$ 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	2.77%
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield	2.99%
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	$ 1,854,056
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	$ 1,785,646