Business Segment Reporting - Core Earnings - Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 40,112		$ 45,058	$ 123,765	$ 132,220
Less: reconciling adjustments			0		0
Guarantee and commitment fees	3,349		3,490	10,265	10,470
Other income/(expense)	(6,781)		949	2,725	588
Non-interest (loss)/income	(3,432)		4,439	12,990	11,058
Provision for loan losses	(760)		(99)	(1,074)	(92)
Release of reserve for losses	137		102	424	(77)
Other non-interest expense	(13,595)		(11,752)	(38,695)	(36,136)
Non-interest expense	(13,458)		(11,650)	(38,271)	(36,213)
Core earnings before income taxes	22,462		37,748	97,410	106,973
Income tax (expense)/benefit	(4,629)		(7,979)	(20,362)	(21,749)
Core earnings before preferred stock dividends	17,833		29,769	77,048	85,224
Preferred stock dividends	(3,427)		(3,295)	(10,508)	(9,886)
Loss on retirement of preferred stock	0	$ (1,956)	0	(1,956)	0
Segment core earnings/(losses)	14,406		26,474	64,584	75,338
Total assets at carrying value	21,315,234		18,474,034	21,315,234	18,474,034	$ 18,694,328
Total on- and off-balance sheet program assets at principal balance	20,932,359		19,540,586	20,932,359	19,540,586
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	15,345		16,425	46,424	47,255
Less: reconciling adjustments	(2,164)		(2,538)	(7,171)	(7,481)
Net effective spread	13,181		13,887	39,253	39,774
Guarantee and commitment fees	4,523		4,489	13,861	13,356
Other income/(expense)	390		294	1,058	1,193
Non-interest (loss)/income	4,913		4,783	14,919	14,549
Provision for loan losses	(760)		(99)	(1,074)	(92)
Release of reserve for losses	137		102	424	(77)
Other non-interest expense	(5,062)		(4,456)	(14,448)	(13,930)
Non-interest expense	(4,925)		(4,354)	(14,024)	(14,007)
Core earnings before income taxes	12,409		14,217	39,074	40,224
Income tax (expense)/benefit	(2,606)		(2,986)	(8,206)	(8,447)
Core earnings before preferred stock dividends	9,803		11,231	30,868	31,777
Preferred stock dividends	0		0	0	0
Loss on retirement of preferred stock				0
Segment core earnings/(losses)	9,803		11,231	30,868	31,777
Total assets at carrying value	4,934,887		4,438,128	4,934,887	4,438,128
Total on- and off-balance sheet program assets at principal balance	7,393,728		7,072,018	7,393,728	7,072,018
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	4,491		5,304	13,045	15,446
Less: reconciling adjustments	(177)		(677)	(670)	(2,021)
Net effective spread	4,314		4,627	12,375	13,425
Guarantee and commitment fees	250		214	712	570
Other income/(expense)	92		5	92	18
Non-interest (loss)/income	342		219	804	588
Provision for loan losses	0		0	0	0
Release of reserve for losses	0		0	0	0
Other non-interest expense	(1,506)		(1,288)	(4,279)	(3,793)
Non-interest expense	(1,506)		(1,288)	(4,279)	(3,793)
Core earnings before income taxes	3,150		3,558	8,900	10,220
Income tax (expense)/benefit	(662)		(747)	(1,870)	(2,146)
Core earnings before preferred stock dividends	2,488		2,811	7,030	8,074
Preferred stock dividends	0		0	0	0
Loss on retirement of preferred stock				0
Segment core earnings/(losses)	2,488		2,811	7,030	8,074
Total assets at carrying value	2,238,558		2,212,515	2,238,558	2,212,515
Total on- and off-balance sheet program assets at principal balance	2,567,763		2,471,251	2,567,763	2,471,251
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,602		3,081	6,264	8,931
Less: reconciling adjustments	1,900		(204)	5,467	(181)
Net effective spread	4,502		2,877	11,731	8,750
Guarantee and commitment fees	348		376	1,069	1,227
Other income/(expense)	17		15	31	25
Non-interest (loss)/income	365		391	1,100	1,252
Provision for loan losses	0		0	0	0
Release of reserve for losses	0		0	0	0
Other non-interest expense	(913)		(732)	(2,595)	(2,144)
Non-interest expense	(913)		(732)	(2,595)	(2,144)
Core earnings before income taxes	3,954		2,536	10,236	7,858
Income tax (expense)/benefit	(830)		(533)	(2,149)	(1,651)
Core earnings before preferred stock dividends	3,124		2,003	8,087	6,207
Preferred stock dividends	0		0	0	0
Loss on retirement of preferred stock				0
Segment core earnings/(losses)	3,124		2,003	8,087	6,207
Total assets at carrying value	1,692,835		956,204	1,692,835	956,204
Total on- and off-balance sheet program assets at principal balance	2,232,602		1,632,037	2,232,602	1,632,037
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	14,853		17,600	49,425	54,237
Less: reconciling adjustments	2,954		(1,958)	2,126	(8,551)
Net effective spread	17,807		15,642	51,551	45,686
Guarantee and commitment fees	87		91	261	271
Other income/(expense)	0		0	0	0
Non-interest (loss)/income	87		91	261	271
Provision for loan losses	0		0	0	0
Release of reserve for losses	0		0	0	0
Other non-interest expense	(2,277)		(1,844)	(6,470)	(5,720)
Non-interest expense	(2,277)		(1,844)	(6,470)	(5,720)
Core earnings before income taxes	15,617		13,889	45,342	40,237
Income tax (expense)/benefit	(3,280)		(2,917)	(9,522)	(8,450)
Core earnings before preferred stock dividends	12,337		10,972	35,820	31,787
Preferred stock dividends	0		0	0	0
Loss on retirement of preferred stock				0
Segment core earnings/(losses)	12,337		10,972	35,820	31,787
Total assets at carrying value	8,651,264		8,103,181	8,651,264	8,103,181
Total on- and off-balance sheet program assets at principal balance	8,738,266		8,365,280	8,738,266	8,365,280
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,821		2,648	8,607	6,351
Less: reconciling adjustments	(164)		(604)	(900)	(1,646)
Net effective spread	2,657		2,044	7,707	4,705
Guarantee and commitment fees	0		0	0	0
Other income/(expense)	(110)		(245)	494	(594)
Non-interest (loss)/income	(110)		(245)	494	(594)
Provision for loan losses	0		0	0	0
Release of reserve for losses	0		0	0	0
Other non-interest expense	(3,837)		(3,432)	(10,903)	(10,549)
Non-interest expense	(3,837)		(3,432)	(10,903)	(10,549)
Core earnings before income taxes	(1,290)		(1,633)	(2,702)	(6,438)
Income tax (expense)/benefit	360		292	663	2,067
Core earnings before preferred stock dividends	(930)		(1,341)	(2,039)	(4,371)
Preferred stock dividends	(3,427)		(3,295)	(10,508)	(9,886)
Loss on retirement of preferred stock				0
Segment core earnings/(losses)	(4,357)		(4,636)	(12,547)	(14,257)
Total assets at carrying value	3,797,690		2,764,006	3,797,690	2,764,006
Total on- and off-balance sheet program assets at principal balance	0		0	0	0
Segment Reconciling Items [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	0		0	0	0
Less: reconciling adjustments	(2,349)		5,981	1,148	19,880
Net effective spread	(2,349)		5,981	1,148	19,880
Guarantee and commitment fees	(1,859)		(1,680)	(5,638)	(4,954)
Other income/(expense)	(7,170)		880	1,050	(54)
Non-interest (loss)/income	(9,029)		(800)	(4,588)	(5,008)
Provision for loan losses	0		0	0	0
Release of reserve for losses	0		0	0	0
Other non-interest expense	0		0	0	0
Non-interest expense	0		0	0	0
Core earnings before income taxes	(11,378)		5,181	(3,440)	14,872
Income tax (expense)/benefit	2,389		(1,088)	722	(3,122)
Core earnings before preferred stock dividends	(8,989)		4,093	(2,718)	11,750
Preferred stock dividends	0		0	0	0
Loss on retirement of preferred stock				(1,956)
Segment core earnings/(losses)	(8,989)		4,093	(4,674)	11,750
Total assets at carrying value	0		0	0	0
Total on- and off-balance sheet program assets at principal balance	$ 0		$ 0	$ 0	$ 0