Financial Derivatives Hedged Items in Fair Value Hedging Relationships (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
HedgedItemsinFairValueHedgingRelationship [Line Items]
TotalAmountsofLineItemsPresentedintheConsolidatedStatementofIncome	$ 30,563	$ 31,000	$ 92,851	$ 94,060
Hedged Liability Discontinued Fair Value Cumulative Increase Decrease	400		400
Loans [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	166,754		166,754		$ 149,304
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	(13,150)		(13,150)		(189)
Long-term Debt [Member]
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	(2,057,594)		(2,057,594)		(1,552,935)
Hedged Liability Fair Value Hedge Cumulative Increase Decrease	22,428		22,428		5,836
Farmer Mac Guaranteed Securities:
HedgedItemsinFairValueHedgingRelationship [Line Items]
Derivative, Amount of Hedged Item	2,514,978		2,514,978		1,928,220
Hedged Asset Fair Value Hedge Cumulative Increase Decrease	$ (65,596)		$ (65,596)		$ (22,853)