Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash	$ 700,000								$ 700,000
Farmer Mac Guaranteed Securities and USDA Securities									290,953,000	$ 203,796,000	$ 150,281,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,955,745,000				$ 8,860,296,000				9,955,745,000	8,860,296,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,487,000				7,093,000				7,487,000	7,093,000
Financial derivatives, at fair value	(19,633,000)				(26,599,000)				(19,633,000)	(26,599,000)
Derivative, Collateral [Abstract]
Collateral Pledged	47,018,000				24,926,000				47,018,000	24,926,000
Derivative Asset	5,709,000				7,093,000				5,709,000	7,093,000
Loans									198,152,000	162,150,000	134,577,000
Total interest expense	(104,237,000)	$ (97,557,000)	$ (91,737,000)	$ (76,317,000)	(70,088,000)	$ (64,935,000)	$ (58,316,000)	$ (49,546,000)	(369,848,000)	(242,885,000)	(171,626,000)
Gain (Loss) on Derivative Instruments, Net, Pretax									3,687,000	(753,000)	(2,311,000)
TotalAmountsofLineItemsPresentedintheConsolidatedStatementofIncome									115,570,000	123,814,000	115,543,000
Derivative Liability	27,385,000				1,673,000				27,385,000	1,673,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,900,000,000				8,800,000,000				9,900,000,000	8,800,000,000
Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Income_Expense related to interest settlements on fair value hedges									27,253,000	26,295,000	20,859,000
Gain (Loss) on Fair Value Hedges Recognized in Earnings									4,941,000	(719,000)	5,043,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness										(320,000)	(353,000)
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										100,000	(5,200,000)
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements									(6,764,000)	(8,845,000)	(16,373,000)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net										(600,000)	(200,000)
Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Premium_Discount recognized on hedged item									(6,000)	(5,000)	(1,000)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									316,000	(1,974,000)	(2,126,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item									(9,182,000)	(4,133,000)	(1,437,000)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments									(8,872,000)	(6,432,000)	(3,917,000)
Cash [Member]
Derivative [Line Items]
Derivative, Collateral, Obligation to Return Cash					0					0
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	1,778,000								1,778,000
Collateral Pledged	0				100,000				0	100,000
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements									34,685,000	35,485,000	37,232,000
Premium_Discount recognized on hedged item									(668,000)	(345,000)	0
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	3,097,084,000				2,086,347,000				3,097,084,000	2,086,347,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	3,004,000				5,240,000				3,004,000	5,240,000
Financial derivatives, at fair value	$ (4,326,000)				$ (5,990,000)				$ (4,326,000)	$ (5,990,000)
Derivative, Average Fixed Interest Rate	2.42%				1.88%				2.42%	1.88%
Derivative, Average Variable Interest Rate	2.58%				1.40%				2.58%	1.40%
Derivative, Average Remaining Maturity									9 years 8 months 29 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	$ 2,031,200,000				$ 1,559,700,000				$ 2,031,200,000	$ 1,559,700,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	885,000				110,000				885,000	110,000
Financial derivatives, at fair value	$ (4,512,000)				$ (4,033,000)				$ (4,512,000)	$ (4,033,000)
Derivative, Average Fixed Interest Rate	2.49%				1.38%				2.49%	1.38%
Derivative, Average Variable Interest Rate	1.94%				1.45%				1.94%	1.45%
Derivative, Average Remaining Maturity									1 year 8 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	$ 373,000,000				$ 365,500,000				$ 373,000,000	$ 365,500,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	2,441,000				1,402,000				2,441,000	1,402,000
Financial derivatives, at fair value	$ (99,000)				$ (138,000)				$ (99,000)	$ (138,000)
Derivative, Average Fixed Interest Rate	2.40%				2.16%				2.40%	2.16%
Derivative, Average Variable Interest Rate	2.83%				1.74%				2.83%	1.74%
Derivative, Average Remaining Maturity									6 years 1 month 12 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	$ 60,000				$ 16,000				$ 60,000	$ 16,000
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	21,000				56,000				21,000	56,000
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax										(1,792,000)	2,379,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense									(10,920,000)	(10,200,000)	(11,480,000)
Gain (Loss) on Derivative Instruments, Net, Pretax									(7,206,000)	(12,240,000)	(9,489,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	316,664,000				345,333,000				316,664,000	345,333,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	796,000				339,000				796,000	339,000
Financial derivatives, at fair value	$ (10,399,000)				$ (16,352,000)				$ (10,399,000)	$ (16,352,000)
Derivative, Average Fixed Interest Rate	3.69%				3.79%				3.69%	3.79%
Derivative, Average Variable Interest Rate	2.52%				1.40%				2.52%	1.40%
Derivative, Average Remaining Maturity									6 years 2 months 29 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	$ 2,347,371,000				$ 3,409,916,000				$ 2,347,371,000	$ 3,409,916,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0				0				0	0
Financial derivatives, at fair value	$ 0				$ 0				$ 0	$ 0
Derivative, Average Fixed Interest Rate	2.10%				1.24%				2.10%	1.24%
Derivative, Average Variable Interest Rate	2.37%				1.25%				2.37%	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	$ 1,770,026,000				$ 1,053,500,000				$ 1,770,026,000	$ 1,053,500,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	421,000				18,000				421,000	18,000
Financial derivatives, at fair value	$ (130,000)				$ (106,000)				$ (130,000)	$ (106,000)
Derivative, Average Variable Interest Rate	2.45%				1.33%				2.45%	1.33%
Derivative, Variable Interest Rate	2.49%				1.42%				2.49%	1.42%
Derivative, Average Remaining Maturity									1 year 3 months 7 days	10 months 28 days
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax										$ 588,000	226,000
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 20,400,000				$ 40,000,000				$ 20,400,000	40,000,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0				0				0	0
Financial derivatives, at fair value	$ (188,000)				$ (36,000)				$ (188,000)	$ (36,000)
Derivative, Weighted-Average Forward Price									$ 121.09	$ 123.96
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax									$ (27,000)	$ (340,000)	162,000
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									1,181,000	0	0
Interest Income [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements									1,861,000	(10,346,000)	(15,494,000)
Income_Expense related to interest settlements on fair value hedges									67,099,000	36,043,000	19,675,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness										0	0
Interest Income [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Premium_Discount recognized on hedged item									0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item									0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments									0	0	0
Interest Income [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements									65,238,000	46,389,000	35,169,000
Premium_Discount recognized on hedged item									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
Gain (Loss) on Derivative Instruments, Net, Pretax									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
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									(212,000)	0	0
Loans and Finance Receivables [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements									(630,000)	(1,141,000)	(1,011,000)
Income_Expense related to interest settlements on fair value hedges									5,654,000	2,238,000	1,052,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness										0	0
Loans and Finance Receivables [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Premium_Discount recognized on hedged item									0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item									0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments									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									6,284,000	3,379,000	2,063,000
Premium_Discount recognized on hedged item									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
Gain (Loss) on Derivative Instruments, Net, Pretax									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
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									3,972,000	0	0
Interest Expense [Member] | Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements									(7,995,000)	2,642,000	132,000
Income_Expense related to interest settlements on fair value hedges									(45,500,000)	(11,986,000)	132,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness										0	0
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Premium_Discount recognized on hedged item									(6,000)	(5,000)	(1,000)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									316,000	(1,974,000)	(2,126,000)
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item									(9,182,000)	(4,133,000)	(1,437,000)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments									(8,872,000)	(6,112,000)	(3,564,000)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Hedged Item Net Interest Settlements									(36,837,000)	(14,283,000)	0
Premium_Discount recognized on hedged item									(668,000)	(345,000)	0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax										0	0
Interest Expense [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Collateral [Abstract]
Total interest expense									0	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax									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
Gain (Loss) on Derivative Instruments [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax									3,687,000	(1,792,000)	2,379,000
Gain (Loss) on Derivative Instruments [Member] | Fair Value Hedging [Member]
Derivative, Collateral [Abstract]
Derivative Net Interest Settlements									0	0	0
Income_Expense related to interest settlements on fair value hedges									0	0	0
Gain (Loss) on Fair Value Hedges Recognized in Earnings									0	(719,000)	5,043,000
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness										(320,000)	(353,000)
Gain (Loss) on Derivative Instruments [Member] | Cash Flow Hedging [Member]
Derivative, Collateral [Abstract]
Premium_Discount recognized on hedged item									0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									0	0	0
Derivative Net Hedge Ineffectiveness Gain Loss Component Recognized On Hedged Item									0	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments									0	(320,000)	(353,000)
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
Premium_Discount recognized on hedged item									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									(10,920,000)	(10,200,000)	(11,480,000)
Gain (Loss) on Derivative Instruments, Net, Pretax									(7,206,000)	(12,240,000)	(9,489,000)
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										588,000	226,000
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative, Collateral [Abstract]
Gain (Loss) on Derivative Instruments, Net, Pretax									$ (27,000)	$ (340,000)	$ 162,000