Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative [Line Items]
Farmer Mac Guaranteed Securities and USDA Securities	$ 62,430	$ 42,522
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,734,245		$ 8,860,296
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	5,142		7,093
Financial derivatives, at fair value	(22,570)		(26,599)
Derivative, Collateral [Abstract]
Collateral Pledged	25,253		24,926
Derivative Asset	5,142		7,093
Loans	45,653	36,852
Total interest expense	(76,317)	(49,546)
Gain (Loss) on Derivative Instruments, Net, Pretax	3,850	(2,486)
TotalAmountsofLineItemsPresentedintheConsolidatedStatementofIncome	27,916	32,314
Derivative Liability	2,683		1,673
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,700,000		8,800,000
Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	2,838	(3,878)
Income_Expense related to interest settlements on fair value hedges	5,450	5,264
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	17,221	1,526
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(14,383)	(5,404)
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		(29)
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(1,491)	(3,192)
Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(267)	(512)
Premium_Discount recognized on hedged item	(2)	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(1,780)	(652)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	(2,049)	(1,193)
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0		0
Collateral Pledged	400		100
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	7,096	8,514
Premium_Discount recognized on hedged item	(155)	(58)
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,323,209		2,086,347
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	804		5,240
Financial derivatives, at fair value	$ (5,763)		$ (5,990)
Derivative, Average Fixed Interest Rate	2.09%		1.88%
Derivative, Average Variable Interest Rate	1.82%		1.40%
Derivative, Average Remaining Maturity	9 years 5 days		5 years 5 months 17 days
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 2,121,700		$ 1,559,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	444		110
Financial derivatives, at fair value	$ (3,956)		$ (4,033)
Derivative, Average Fixed Interest Rate	1.77%		1.38%
Derivative, Average Variable Interest Rate	1.58%		1.45%
Derivative, Average Remaining Maturity	1 year 9 months 5 days		1 year 8 months 5 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 380,500		$ 365,500
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	2,878		1,402
Financial derivatives, at fair value	$ (261)		$ (138)
Derivative, Average Fixed Interest Rate	2.20%		2.16%
Derivative, Average Variable Interest Rate	2.09%		1.74%
Derivative, Average Remaining Maturity	5 years 9 months 25 days		5 years 10 months 3 days
Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	$ 50		$ 16
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	18		56
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		(6,393)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	4,075	(6,684)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	330,709		345,333
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	1,062		339
Financial derivatives, at fair value	$ (11,725)		$ (16,352)
Derivative, Average Fixed Interest Rate	3.72%		3.79%
Derivative, Average Variable Interest Rate	1.81%		1.40%
Derivative, Average Remaining Maturity	6 years 9 months 5 days		6 years 8 months 5 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 3,256,427		$ 3,409,916
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0		0
Financial derivatives, at fair value	$ 0		$ 0
Derivative, Average Fixed Interest Rate	1.60%		1.24%
Derivative, Average Variable Interest Rate	1.73%		1.25%
Derivative, Average Remaining Maturity	10 months 10 days		11 months
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,278,500		$ 1,053,500
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	4		18
Financial derivatives, at fair value	$ (517)		$ (106)
Derivative, Average Variable Interest Rate	1.75%		1.33%
Derivative, Variable Interest Rate	1.74%		1.42%
Derivative, Average Remaining Maturity	1 year 2 months 15 days		10 months 28 days
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 0	399
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	43,200		$ 40,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0		0
Financial derivatives, at fair value	$ (366)		$ (36)
Derivative, Weighted-Average Forward Price	$ 120.29		$ 123.96
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ (225)	(108)
Interest Income [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0
Interest Income [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	1,501	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(18,948)
Interest Income [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(1,488)	(3,158)
Income_Expense related to interest settlements on fair value hedges	12,109	7,117
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	20,449
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0
Interest Income [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	0
Premium_Discount recognized on hedged item	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	0
Interest Income [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	13,597	10,275
Premium_Discount recognized on hedged item	0	0
Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0
Interest Income [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Interest Income [Member] | Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Interest Income [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Loans and Finance Receivables [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0
Loans and Finance Receivables [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	(153)	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(6,572)
Loans and Finance Receivables [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(297)	(217)
Income_Expense related to interest settlements on fair value hedges	1,117	299
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	6,419
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0
Loans and Finance Receivables [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	0
Premium_Discount recognized on hedged item	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	0
Loans and Finance Receivables [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	1,414	516
Premium_Discount recognized on hedged item	0	0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Interest Expense [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0
Interest Expense [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	1,490	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	11,137
Interest Expense [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	294	183
Income_Expense related to interest settlements on fair value hedges	(7,776)	(2,152)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	(9,647)
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(267)	(512)
Premium_Discount recognized on hedged item	(2)	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(1,780)	(652)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	(2,049)	(1,164)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	(7,915)	(2,277)
Premium_Discount recognized on hedged item	(155)	(58)
Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0
Gain (Loss) on Derivative Instruments [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	3,850	(6,393)
Gain (Loss) on Derivative Instruments [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	0	0
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	(3,878)
Income_Expense related to interest settlements on fair value hedges	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	0	1,526
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	(5,404)
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		(29)
Gain (Loss) on Derivative Instruments [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	0
Premium_Discount recognized on hedged item	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	(29)
Gain (Loss) on Derivative Instruments [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	0	0
Premium_Discount recognized on hedged item	0	0
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	4,075	(6,684)
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0	399
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ (225)	$ (108)