Business Segment Reporting - Core Earnings - Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 43,054	$ 43,933	$ 83,653	$ 87,162
Guarantee and commitment fees	3,403	3,481	6,916	6,980
Other income/(expense)	9,329	2,899	9,506	(361)
Non-interest income	12,732	6,380	16,422	6,619
Provision for loan losses	(578)	(424)	(314)	7
Release of reserve for losses	158	(158)	287	(179)
Other non-interest expense	(12,210)	(12,763)	(25,100)	(24,384)
Non-interest expense	(12,052)	(12,921)	(24,813)	(24,563)
Core earnings before income taxes	43,156	36,968	74,948	69,225
Income tax (expense)/benefit	(9,111)	(7,332)	(15,733)	(13,770)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	34,045	29,636	59,215	55,455
Preferred stock dividends	(3,785)	(3,296)	(7,081)	(6,591)
Loss on retirement of preferred stock	(1,956)	0	(1,956)	0
Segment core earnings/(losses)	28,304	26,340	50,178	48,864
Total assets at carrying value	20,738,160	18,626,771	20,738,160	18,626,771	$ 18,694,328
Total on- and off-balance sheet program assets at principal balance	20,746,735	19,524,837	20,746,735	19,524,837
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	15,797	15,889	31,079	30,830
Less: reconciling adjustments	(2,462)	(2,542)	(5,007)	(4,943)
Net effective spread	13,335	13,347	26,072	25,887
Guarantee and commitment fees	4,594	4,488	9,338	8,867
Other income/(expense)	188	341	668	899
Non-interest income	4,782	4,829	10,006	9,766
Provision for loan losses	(578)	(424)	(314)	7
Release of reserve for losses	158	(158)	287	(179)
Other non-interest expense	(4,587)	(4,954)	(9,386)	(9,474)
Non-interest expense	(4,429)	(5,112)	(9,099)	(9,653)
Core earnings before income taxes	13,110	12,640	26,665	26,007
Income tax (expense)/benefit	(2,753)	(2,654)	(5,600)	(5,461)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	10,357	9,986	21,065	20,546
Preferred stock dividends	0	0	0	0
Loss on retirement of preferred stock	0		0
Segment core earnings/(losses)	10,357	9,986	21,065	20,546
Total assets at carrying value	4,872,766	4,428,172	4,872,766	4,428,172
Total on- and off-balance sheet program assets at principal balance	7,291,352	7,045,397	7,291,352	7,045,397
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	4,112	5,072	8,554	10,142
Less: reconciling adjustments	(15)	(674)	(493)	(1,344)
Net effective spread	4,097	4,398	8,061	8,798
Guarantee and commitment fees	238	190	462	356
Other income/(expense)	0	8	0	13
Non-interest income	238	198	462	369
Provision for loan losses	0	0	0	0
Release of reserve for losses	0	0	0	0
Other non-interest expense	(1,345)	(1,312)	(2,773)	(2,505)
Non-interest expense	(1,345)	(1,312)	(2,773)	(2,505)
Core earnings before income taxes	2,990	3,284	5,750	6,662
Income tax (expense)/benefit	(628)	(690)	(1,208)	(1,399)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	2,362	2,594	4,542	5,263
Preferred stock dividends	0	0	0	0
Loss on retirement of preferred stock	0		0
Segment core earnings/(losses)	2,362	2,594	4,542	5,263
Total assets at carrying value	2,198,514	2,177,345	2,198,514	2,177,345
Total on- and off-balance sheet program assets at principal balance	2,521,394	2,418,115	2,521,394	2,418,115
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	3,936	3,313	3,662	5,850
Less: reconciling adjustments	60	(390)	3,567	23
Net effective spread	3,996	2,923	7,229	5,873
Guarantee and commitment fees	358	402	721	851
Other income/(expense)	7	5	14	10
Non-interest income	365	407	735	861
Provision for loan losses	0	0	0	0
Release of reserve for losses	0	0	0	0
Other non-interest expense	(816)	(739)	(1,682)	(1,412)
Non-interest expense	(816)	(739)	(1,682)	(1,412)
Core earnings before income taxes	3,545	2,591	6,282	5,322
Income tax (expense)/benefit	(744)	(544)	(1,319)	(1,118)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	2,801	2,047	4,963	4,204
Preferred stock dividends	0	0	0	0
Loss on retirement of preferred stock	0		0
Segment core earnings/(losses)	2,801	2,047	4,963	4,204
Total assets at carrying value	1,580,979	995,068	1,580,979	995,068
Total on- and off-balance sheet program assets at principal balance	2,155,671	1,669,440	2,155,671	1,669,440
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	16,385	18,805	34,572	36,637
Less: reconciling adjustments	986	(3,585)	(828)	(6,593)
Net effective spread	17,371	15,220	33,744	30,044
Guarantee and commitment fees	86	91	174	180
Other income/(expense)	0	0	0	0
Non-interest income	86	91	174	180
Provision for loan losses	0	0	0	0
Release of reserve for losses	0	0	0	0
Other non-interest expense	(2,034)	(2,030)	(4,193)	(3,876)
Non-interest expense	(2,034)	(2,030)	(4,193)	(3,876)
Core earnings before income taxes	15,423	13,281	29,725	26,348
Income tax (expense)/benefit	(3,239)	(2,789)	(6,242)	(5,533)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	12,184	10,492	23,483	20,815
Preferred stock dividends	0	0	0	0
Loss on retirement of preferred stock	0		0
Segment core earnings/(losses)	12,184	10,492	23,483	20,815
Total assets at carrying value	8,633,059	8,144,763	8,633,059	8,144,763
Total on- and off-balance sheet program assets at principal balance	8,778,318	8,391,885	8,778,318	8,391,885
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,824	854	5,786	3,703
Less: reconciling adjustments	(268)	(580)	(736)	(1,042)
Net effective spread	2,556	274	5,050	2,661
Guarantee and commitment fees	0	0	0	0
Other income/(expense)	582	(209)	604	(349)
Non-interest income	582	(209)	604	(349)
Provision for loan losses	0	0	0	0
Release of reserve for losses	0	0	0	0
Other non-interest expense	(3,428)	(3,728)	(7,066)	(7,117)
Non-interest expense	(3,428)	(3,728)	(7,066)	(7,117)
Core earnings before income taxes	(290)	(3,663)	(1,412)	(4,805)
Income tax (expense)/benefit	13	1,200	303	1,775
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	(277)	(2,463)	(1,109)	(3,030)
Preferred stock dividends	(3,785)	(3,296)	(7,081)	(6,591)
Loss on retirement of preferred stock	0		0
Segment core earnings/(losses)	(4,062)	(5,759)	(8,190)	(9,621)
Total assets at carrying value	3,452,842	2,881,423	3,452,842	2,881,423
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	1,699	7,771	3,497	13,899
Net effective spread	1,699	7,771	3,497	13,899
Guarantee and commitment fees	(1,873)	(1,690)	(3,779)	(3,274)
Other income/(expense)	8,552	2,754	8,220	(934)
Non-interest income	6,679	1,064	4,441	(4,208)
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	8,378	8,835	7,938	9,691
Income tax (expense)/benefit	(1,760)	(1,855)	(1,667)	(2,034)
Core earnings before preferred stock dividends and attribution of income to non-controlling interest	6,618	6,980	6,271	7,657
Preferred stock dividends	0	0	0	0
Loss on retirement of preferred stock	(1,956)		(1,956)
Segment core earnings/(losses)	4,662	6,980	4,315	7,657
Total assets at carrying value	0	0	0	0
Total on- and off-balance sheet program assets at principal balance	$ 0	$ 0	$ 0	$ 0