Financial Derivatives - Schedule of Net Income/(Expense) Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income Farmer Mac Guaranteed Securities and USDA Securities	$ 81,649	$ 76,870	$ 252,629	$ 213,479
Interest Income Loans	56,992	50,622	167,792	145,671
Total Interest Expense	(121,384)	(97,557)	(358,374)	(265,611)
Gains/(losses) on financial derivatives	(7,360)	628	1,193	(688)
Total	9,897	30,563	63,240	92,851
Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	116,623	(15,090)	316,799	(39,602)
(Losses)/gains on fair value hedging relationships	(4,490)	1,051	(8,617)	5,751
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Discount amortization recognized on hedged items	(1)	(2)	(3)	(5)
Interest settlements reclassified from AOCI into net income on derivatives	317	189	1,260	(21)
Recognized on hedged items	(2,726)	(2,501)	(8,142)	(6,611)
Expense recognized on cash flow hedges	(2,410)	(2,314)	(6,885)	(6,637)
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	27,978	8,608	72,233	22,275
Discount amortization recognized on hedged items	(146)	(177)	(460)	(534)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	(7,360)	628	1,193	(688)
Interest Income [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	84,164	(13,432)	258,155	(43,229)
(Losses)/gains on fair value hedging relationships	(3,331)	(12)	(4,731)	3,125
Interest Income [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Discount amortization recognized on hedged items	0	0	0	0
Interest settlements reclassified from AOCI into net income on derivatives	0	0	0	0
Recognized on hedged items	0	0	0	0
Expense recognized on cash flow hedges	0	0	0	0
Interest Income [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	31,435	16,769	86,628	46,289
Discount amortization recognized on hedged items	0	0	0	0
Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Loans and Finance Receivables [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	33,493	(4,062)	83,524	(13,106)
(Losses)/gains on fair value hedging relationships	(2,104)	(153)	(6,107)	(542)
Loans and Finance Receivables [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Discount amortization recognized on hedged items	0	0	0	0
Interest settlements reclassified from AOCI into net income on derivatives	0	0	0	0
Recognized on hedged items	0	0	0	0
Expense recognized on cash flow hedges	0	0	0	0
Loans and Finance Receivables [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	7,321	1,660	18,199	4,619
Discount amortization recognized on hedged items	0	0	0	0
Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Interest Expense [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	(1,034)	2,404	(24,880)	16,733
(Losses)/gains on fair value hedging relationships	945	1,216	2,221	3,168
Interest Expense [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Discount amortization recognized on hedged items	(1)	(2)	(3)	(5)
Interest settlements reclassified from AOCI into net income on derivatives	317	189	1,260	(21)
Recognized on hedged items	(2,726)	(2,501)	(8,142)	(6,611)
Expense recognized on cash flow hedges	(2,410)	(2,314)	(6,885)	(6,637)
Interest Expense [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	(10,778)	(9,821)	(32,594)	(28,633)
Discount amortization recognized on hedged items	(146)	(177)	(460)	(534)
Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Gain (Loss) on Derivative Instruments [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	0	0	0	0
(Losses)/gains on fair value hedging relationships	0	0	0	0
Gain (Loss) on Derivative Instruments [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Discount amortization recognized on hedged items	0	0	0	0
Interest settlements reclassified from AOCI into net income on derivatives	0	0	0	0
Recognized on hedged items	0	0	0	0
Expense recognized on cash flow hedges	0	0	0	0
Gain (Loss) on Derivative Instruments [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on hedged items	0	0	0	0
Discount amortization recognized on hedged items	0	0	0	0
Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	(7,360)	628	1,193	(688)
Interest Rate Swap [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on derivatives	(2,596)	(1,528)	(5,894)	(5,410)
Income/(expense) related to interest settlements on fair value hedging relationships	25,236	6,903	65,879	16,331
Recognized on derivatives	(121,113)	16,141	(325,416)	45,353
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest Expense	127	(3,048)	(3,321)	(8,903)
Gains/(losses) on financial derivatives	(7,402)	3,267	5,920	7,443
Interest Rate Swap [Member] | Interest Income [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on derivatives	(1,051)	1,272	1,665	465
Income/(expense) related to interest settlements on fair value hedging relationships	30,384	18,041	88,293	46,754
Recognized on derivatives	(87,495)	13,420	(262,886)	46,354
Interest Rate Swap [Member] | Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest Expense	0	0	0	0
Gains/(losses) on financial derivatives	0	0	0	0
Interest Rate Swap [Member] | Loans and Finance Receivables [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on derivatives	(584)	(98)	(808)	(560)
Income/(expense) related to interest settlements on fair value hedging relationships	6,737	1,562	17,391	4,059
Recognized on derivatives	(35,597)	3,909	(89,631)	12,564
Interest Rate Swap [Member] | Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest Expense	0	0	0	0
Gains/(losses) on financial derivatives	0	0	0	0
Interest Rate Swap [Member] | Interest Expense [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on derivatives	(961)	(2,702)	(6,751)	(5,315)
Income/(expense) related to interest settlements on fair value hedging relationships	(11,885)	(12,700)	(39,805)	(34,482)
Recognized on derivatives	1,979	(1,188)	27,101	(13,565)
Interest Rate Swap [Member] | Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest Expense	0	0	0	0
Gains/(losses) on financial derivatives	0	0	0	0
Interest Rate Swap [Member] | Gain (Loss) on Derivative Instruments [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Recognized on derivatives	0	0	0	0
Income/(expense) related to interest settlements on fair value hedging relationships	0	0	0	0
Recognized on derivatives	0	0	0	0
Interest Rate Swap [Member] | Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total Interest Expense	127	(3,048)	(3,321)	(8,903)
Gains/(losses) on financial derivatives	(7,402)	3,267	5,920	7,443
Future [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	(85)	409	(1,406)	772
Future [Member] | Interest Income [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Future [Member] | Loans and Finance Receivables [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Future [Member] | Interest Expense [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	0	0	0	0
Future [Member] | Gain (Loss) on Derivative Instruments [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains/(losses) on financial derivatives	$ (85)	$ 409	$ (1,406)	$ 772