Business Segment Reporting - Core Earnings - Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 49,370	$ 40,112	$ 43,054	$ 40,599	$ 42,216	$ 45,058	$ 43,933	$ 43,229	$ 173,135	$ 174,436	$ 157,647
Less: reconciling adjustments									0	0	0
Net effective spread									0	0	0
Guarantee and commitment fees	3,401	3,349	3,403	3,513	3,506	3,490	3,481	3,499	13,666	13,976	14,114
Other income/(expense)									7,276	(2,236)	3,398
Non-interest income	7,952	(3,432)	12,732	3,690	682	4,439	6,380	239	20,942	11,740	17,512
Provision for loan losses	(2,430)	(760)	(578)	264	(146)	(99)	(424)	431	(3,504)	(238)	(1,708)
Release of reserve for losses									3	(97)	(50)
Other non-interest expense									(51,925)	(49,819)	(42,715)
Non-interest expense	(13,651)	(13,458)	(12,052)	(12,761)	(13,703)	(11,650)	(12,921)	(11,642)	(51,922)	(49,916)	(42,765)
Core earnings before income taxes									138,651	136,022	130,686
Income tax (expense)/benefit	(8,743)	(4,629)	(9,111)	(6,622)	(6,193)	(7,979)	(7,332)	(6,438)	(29,105)	(27,942)	(46,369)
Core earnings before preferred stock dividends									109,546	108,080	84,317
Preferred stock dividends	(3,432)	(3,427)	(3,785)	(3,296)	(3,296)	$ (3,295)	$ (3,296)	$ (3,295)	(13,940)	(13,182)	(13,182)
Non-controlling interest									0	0	165
Loss on retirement of preferred stock	0	$ 0	$ (1,956)	$ 0					(1,956)	0	0
Segment core earnings/(losses)									93,650	94,898	71,300
Total assets at carrying value	21,709,374				18,694,328				21,709,374	18,694,328	17,792,274
Total on- and off-balance sheet program assets at principal balance	21,117,942				19,724,524				21,117,942	19,724,524	19,007,311
Corporate
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									11,069	9,105	10,811
Less: reconciling adjustments									(987)	(2,047)	(1,091)
Net effective spread									10,082	7,058	9,720
Guarantee and commitment fees									0	0	0
Other income/(expense)									166	(913)	171
Non-interest income									166	(913)	171
Provision for loan losses									0	0	0
Release of reserve for losses									0	0	0
Other non-interest expense									(14,505)	(14,411)	(12,908)
Non-interest expense									(14,505)	(14,411)	(12,908)
Core earnings before income taxes									(4,257)	(8,266)	(3,017)
Income tax (expense)/benefit									907	2,361	1,792
Core earnings before preferred stock dividends									(3,350)	(5,905)	(1,225)
Preferred stock dividends									(13,940)	(13,182)	(13,182)
Non-controlling interest											165
Loss on retirement of preferred stock									0
Segment core earnings/(losses)									(17,290)	(19,087)	(14,242)
Total assets at carrying value	3,664,823				2,716,994				3,664,823	2,716,994	2,605,657
Total on- and off-balance sheet program assets at principal balance	0				0				0	0	0
Reconciling Adjustments
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									0	0	0
Less: reconciling adjustments									4,527	23,241	16,344
Net effective spread									4,527	23,241	16,344
Guarantee and commitment fees									(7,669)	(6,756)	(6,236)
Other income/(expense)									5,501	(2,747)	715
Non-interest income									(2,168)	(9,503)	(5,521)
Provision for loan losses									0	0	0
Release of reserve for losses									0	0	0
Other non-interest expense									0	0	0
Non-interest expense									0	0	0
Core earnings before income taxes									2,359	13,738	10,823
Income tax (expense)/benefit									(495)	(2,887)	(5,154)
Core earnings before preferred stock dividends									1,864	10,851	5,669
Preferred stock dividends									0	0	0
Non-controlling interest											0
Loss on retirement of preferred stock									(1,956)
Segment core earnings/(losses)									(92)	10,851	5,669
Total assets at carrying value	0				0				0	0	0
Total on- and off-balance sheet program assets at principal balance	0				0				0	0	0
Farm & Ranch | Operating Segments
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									65,098	62,951	54,290
Less: reconciling adjustments									(9,471)	(9,889)	(8,922)
Net effective spread									55,627	53,062	45,368
Guarantee and commitment fees									18,593	17,976	17,175
Other income/(expense)									1,397	1,371	2,449
Non-interest income									19,990	19,347	19,624
Provision for loan losses									(3,504)	(238)	(1,708)
Release of reserve for losses									3	(97)	(50)
Other non-interest expense									(19,375)	(19,026)	(16,554)
Non-interest expense									(19,372)	(19,123)	(16,604)
Core earnings before income taxes									52,741	53,048	46,680
Income tax (expense)/benefit									(11,076)	(11,140)	(16,338)
Core earnings before preferred stock dividends									41,665	41,908	30,342
Preferred stock dividends									0	0	0
Non-controlling interest											0
Loss on retirement of preferred stock									0
Segment core earnings/(losses)									41,665	41,908	30,342
Total assets at carrying value	5,408,302				4,701,736				5,408,302	4,701,736	4,274,693
Total on- and off-balance sheet program assets at principal balance	7,776,950				7,233,972				7,776,950	7,233,972	6,867,586
USDA Guarantees | Operating Segments
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									17,470	20,554	21,106
Less: reconciling adjustments									(732)	(2,499)	(2,287)
Net effective spread									16,738	18,055	18,819
Guarantee and commitment fees									958	797	456
Other income/(expense)									174	20	43
Non-interest income									1,132	817	499
Provision for loan losses									0	0	0
Release of reserve for losses									0	0	0
Other non-interest expense									(5,757)	(5,309)	(4,384)
Non-interest expense									(5,757)	(5,309)	(4,384)
Core earnings before income taxes									12,113	13,563	14,934
Income tax (expense)/benefit									(2,545)	(2,848)	(5,227)
Core earnings before preferred stock dividends									9,568	10,715	9,707
Preferred stock dividends									0	0	0
Non-controlling interest											0
Loss on retirement of preferred stock									0
Segment core earnings/(losses)									9,568	10,715	9,707
Total assets at carrying value	2,311,932				2,240,906				2,311,932	2,240,906	2,195,189
Total on- and off-balance sheet program assets at principal balance	2,620,175				2,515,620				2,620,175	2,515,620	2,352,214
Rural Utilities | Operating Segments
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									10,459	12,505	11,598
Less: reconciling adjustments									6,143	(922)	(539)
Net effective spread									16,602	11,583	11,059
Guarantee and commitment fees									1,412	1,599	1,914
Other income/(expense)									38	33	20
Non-interest income									1,450	1,632	1,934
Provision for loan losses									0	0	0
Release of reserve for losses									0	0	0
Other non-interest expense									(3,898)	(3,062)	(2,430)
Non-interest expense									(3,898)	(3,062)	(2,430)
Core earnings before income taxes									14,154	10,153	10,563
Income tax (expense)/benefit									(2,972)	(2,133)	(3,696)
Core earnings before preferred stock dividends									11,182	8,020	6,867
Preferred stock dividends									0	0	0
Non-controlling interest											0
Loss on retirement of preferred stock									0
Segment core earnings/(losses)									11,182	8,020	6,867
Total assets at carrying value	1,717,405				945,282				1,717,405	945,282	1,088,986
Total on- and off-balance sheet program assets at principal balance	2,280,571				1,592,115				2,280,571	1,592,115	1,882,633
Institutional Credit | Operating Segments
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									69,039	69,321	59,842
Less: reconciling adjustments									520	(7,884)	(3,505)
Net effective spread									69,559	61,437	56,337
Guarantee and commitment fees									372	360	805
Other income/(expense)									0	0	0
Non-interest income									372	360	805
Provision for loan losses									0	0	0
Release of reserve for losses									0	0	0
Other non-interest expense									(8,390)	(8,011)	(6,439)
Non-interest expense									(8,390)	(8,011)	(6,439)
Core earnings before income taxes									61,541	53,786	50,703
Income tax (expense)/benefit									(12,924)	(11,295)	(17,746)
Core earnings before preferred stock dividends									48,617	42,491	32,957
Preferred stock dividends									0	0	0
Non-controlling interest											0
Loss on retirement of preferred stock									0
Segment core earnings/(losses)									48,617	42,491	32,957
Total assets at carrying value	8,606,912				8,089,410				8,606,912	8,089,410	7,627,749
Total on- and off-balance sheet program assets at principal balance	$ 8,440,246				$ 8,382,817				$ 8,440,246	$ 8,382,817	$ 7,904,878