Financial Derivatives Hedged Items in Fair Value Hedging Relationships (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
HedgedItemsinFairValueHedgingRelationship [Line Items]
TotalAmountsofLineItemsPresentedintheConsolidatedStatementofIncome	$ 34,372	$ 30,746	$ 62,288	$ 63,060
Hedged Liability Discontinued Fair Value Cumulative Increase Decrease	400		400
Loans [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	162,884		162,884		$ 149,304
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	(9,088)		(9,088)		(189)
Long-term Debt [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	(2,345,273)		(2,345,273)		(1,552,935)
Hedged Liability Fair Value Hedge Cumulative Increase Decrease	20,072		20,072		5,836
Farmer Mac Guaranteed Securities:
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	2,223,750		2,223,750		1,928,220
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	$ (52,164)		$ (52,164)		$ (22,853)