Financial Derivatives Hedged Items in Fair Value Hedging Relationships (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
HedgedItemsinFairValueHedgingRelationship [Line Items]
Hedged Liability Discontinued Fair Value Cumulative Increase Decrease	$ 400
Loans [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	161,656	$ 149,304
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	(44)	(189)
Long-term Debt [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	(2,114,705)	(1,552,935)
Hedged Liability Fair Value Hedge Cumulative Increase Decrease	5,805	5,836
Farmer Mac Guaranteed Securities:
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	2,135,593	1,928,220
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	$ (22,367)	$ (22,853)