Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Farmer Mac Guaranteed Securities and USDA Securities	$ 76,870,000	$ 54,350,000	$ 213,479,000	$ 146,978,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,703,115,000		9,703,115,000		$ 8,860,296,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	8,007,000		8,007,000		7,093,000
Financial derivatives, at fair value	(17,841,000)		(17,841,000)		(26,599,000)
Derivative, Collateral [Abstract]
Collateral Pledged	46,841,000		46,841,000		24,926,000
Derivative Asset	8,007,000		8,007,000		7,093,000
Loans	50,622,000	40,924,000	145,671,000	117,349,000
Total interest expense	(97,557,000)	(64,935,000)	(265,611,000)	(172,797,000)
Gain (Loss) on Derivative Instruments, Net, Pretax	(628,000)	(661,000)	688,000	(2,530,000)
TotalAmountsofLineItemsPresentedintheConsolidatedStatementofIncome	30,563,000	31,000,000	92,851,000	94,060,000
Derivative Liability	29,000,000		29,000,000		(1,673,000)
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,700,000,000		9,700,000,000		8,800,000,000
Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	1,051,000	1,742,000	5,751,000	(716,000)
Income_Expense related to interest settlements on fair value hedges	6,903,000	5,851,000	16,331,000	16,253,000
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	16,141,000	1,576,000	45,353,000	(5,466,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(15,090,000)	166,000	(39,602,000)	4,750,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		(191,000)		(365,000)
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Gain (Loss) on Components Included from Assessment of Interest Rate Fair Value Hedge Effectiveness		(1,600,000)		700,000
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(1,528,000)	(1,892,000)	(5,410,000)	(6,861,000)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net		100,000		0
Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	189,000	(454,000)	(21,000)	(1,494,000)
Premium_Discount recognized on hedged item	(2,000)	(1,000)	(5,000)	(3,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(2,501,000)	(1,169,000)	(6,611,000)	(2,665,000)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	(2,314,000)	(1,815,000)	(6,637,000)	(4,527,000)
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0		0		0
Collateral Pledged	0		0		100,000
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	8,608,000	7,864,000	22,275,000	23,425,000
Premium_Discount recognized on hedged item	(177,000)	(121,000)	(534,000)	(311,000)
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,801,164,000		2,801,164,000		2,086,347,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	860,000		860,000		5,240,000
Financial derivatives, at fair value	$ (4,493,000)		$ (4,493,000)		$ (5,990,000)
Derivative, Average Fixed Interest Rate	2.04%		2.04%		1.88%
Derivative, Average Variable Interest Rate	2.13%		2.13%		1.40%
Derivative, Average Remaining Maturity			10 years 4 months 22 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	$ 2,131,200,000		$ 2,131,200,000		$ 1,559,700,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	682,000		682,000		110,000
Financial derivatives, at fair value	$ (4,796,000)		$ (4,796,000)		$ (4,033,000)
Derivative, Average Fixed Interest Rate	2.20%		2.20%		1.38%
Derivative, Average Variable Interest Rate	1.78%		1.78%		1.45%
Derivative, Average Remaining Maturity			1 year 8 months 28 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	$ 389,500,000		$ 389,500,000		$ 365,500,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	4,429,000		4,429,000		1,402,000
Financial derivatives, at fair value	$ (276,000)		$ (276,000)		$ (138,000)
Derivative, Average Fixed Interest Rate	2.36%		2.36%		2.16%
Derivative, Average Variable Interest Rate	2.57%		2.57%		1.74%
Derivative, Average Remaining Maturity			6 years 1 month 6 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	$ 163,000		$ 163,000		$ 16,000
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	6,000		6,000		56,000
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		890,000		(3,611,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense	(3,048,000)	(2,676,000)	(8,903,000)	(8,318,000)
Gain (Loss) on Derivative Instruments, Net, Pretax	(3,267,000)	(1,645,000)	(7,443,000)	(12,324,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	317,586,000		317,586,000		345,333,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	1,966,000		1,966,000		339,000
Financial derivatives, at fair value	$ (8,065,000)		$ (8,065,000)		$ (16,352,000)
Derivative, Average Fixed Interest Rate	3.69%		3.69%		3.79%
Derivative, Average Variable Interest Rate	2.33%		2.33%		1.40%
Derivative, Average Remaining Maturity			6 years 5 months 22 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	$ 2,521,965,000		$ 2,521,965,000		$ 3,409,916,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0		0		0
Financial derivatives, at fair value	$ 0		$ 0		$ 0
Derivative, Average Fixed Interest Rate	1.99%		1.99%		1.24%
Derivative, Average Variable Interest Rate	2.16%		2.16%		1.25%
Derivative, Average Remaining Maturity			9 months 32 days		11 months
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,529,000,000		$ 1,529,000,000		$ 1,053,500,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	233,000		233,000		18,000
Financial derivatives, at fair value	$ (206,000)		$ (206,000)		$ (106,000)
Derivative, Average Variable Interest Rate	2.01%		2.01%		1.33%
Derivative, Variable Interest Rate	2.07%		2.07%		1.42%
Derivative, Average Remaining Maturity			1 year 4 months 28 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		588,000
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 12,700,000		$ 12,700,000		$ 40,000,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0		0		0
Financial derivatives, at fair value	(11,000)		$ (11,000)		$ (36,000)
Derivative, Weighted-Average Forward Price			$ 118.70		$ 123.96
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	(409,000)	(141,000)	$ (772,000)	(193,000)
Interest Income [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0		0
Interest Income [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	(12,000)	0	3,125,000	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(13,432,000)	0	(43,229,000)	0
Interest Income [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	1,272,000	(2,267,000)	465,000	(8,251,000)
Income_Expense related to interest settlements on fair value hedges	18,041,000	9,719,000	46,754,000	25,961,000
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	13,420,000	0	46,354,000	0
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0		0
Interest Income [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	0	0	0
Premium_Discount recognized on hedged item	0	0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	0	0	0
Interest Income [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	16,769,000	11,986,000	46,289,000	34,212,000
Premium_Discount recognized on hedged item	0	0	0	0
Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		0
Interest Income [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense	0	0	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0	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	0	0
Loans and Finance Receivables [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	0		0
Loans and Finance Receivables [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	(153,000)	0	(542,000)	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(4,062,000)	0	(13,106,000)	0
Loans and Finance Receivables [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(98,000)	(342,000)	(560,000)	(767,000)
Income_Expense related to interest settlements on fair value hedges	1,562,000	605,000	4,059,000	1,278,000
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	3,909,000	0	12,564,000	0
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0		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	0	0
Premium_Discount recognized on hedged item	0	0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	0	0	0
Loans and Finance Receivables [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	1,660,000	947,000	4,619,000	2,045,000
Premium_Discount recognized on hedged item	0	0	0	0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense	0	0	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0	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	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	1,216,000	0	3,168,000	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	2,404,000	0	16,733,000	0
Interest Expense [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	(2,702,000)	717,000	(5,315,000)	2,157,000
Income_Expense related to interest settlements on fair value hedges	(12,700,000)	(4,473,000)	(34,482,000)	(10,986,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	(1,188,000)	0	(13,565,000)	0
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		0		0
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	189,000	(454,000)	(21,000)	(1,494,000)
Premium_Discount recognized on hedged item	(2,000)	(1,000)	(5,000)	(3,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	(2,501,000)	(1,169,000)	(6,611,000)	(2,665,000)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	(2,314,000)	(1,624,000)	(6,637,000)	(4,162,000)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements	(9,821,000)	(5,069,000)	(28,633,000)	(12,832,000)
Premium_Discount recognized on hedged item	(177,000)	(121,000)	(534,000)	(311,000)
Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax		0	0	0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense	0	0	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0	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	0	0
Gain (Loss) on Derivative Instruments [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	(628,000)	890,000	688,000	(3,611,000)
Gain (Loss) on Derivative Instruments [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements	0	0	0	0
Gain (Loss) on Fair Value Hedges Recognized in Earnings	0	1,742,000	0	(716,000)
Income_Expense related to interest settlements on fair value hedges	0	0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Derivative	0	1,576,000	0	(5,466,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	166,000	0	4,750,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness		(191,000)		(365,000)
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	0	0
Premium_Discount recognized on hedged item	0	0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item	0	0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0	(191,000)	0	(365,000)
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	0	0
Premium_Discount recognized on hedged item	0	0	0	0
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense	(3,048,000)	(2,676,000)	(8,903,000)	(8,318,000)
Gain (Loss) on Derivative Instruments, Net, Pretax	(3,267,000)	(1,645,000)	(7,443,000)	(12,324,000)
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		588,000
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ (409,000)	$ (141,000)	$ (772,000)	$ (193,000)