Business Segment Reporting - Core Earnings - Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 43,933	$ 39,731	$ 87,162	$ 76,802
Less: reconciling adjustments(1)(2)(3)	0	0	0	0
Net Effective Spread	0	0	0	0
Guaranty Fee Income	3,481	3,472	6,980	7,316
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	2,899	272	(361)	3,224
Non-interest income	6,380	3,744	6,619	10,540
(Provision for)/release of loan losses	(424)	(327)	7	(964)
Provision for Other Credit Losses	(158)	(139)	(179)	54
Noninterest Expense, Excluding Loan, Lease and Other Losses	(12,763)	(11,251)	(24,384)	(21,993)
Non-interest expense	(12,921)	(11,390)	(24,563)	(21,939)
Income Tax Expense (Benefit)	(7,332)	(11,124)	(13,770)	(21,910)
Net income	29,636	20,634	55,455	42,529
Net income	36,968	31,758	69,225	64,439
Preferred Stock Dividends, Income Statement Impact	(3,296)	(3,296)	(6,591)	(6,591)
Less: Net loss attributable to non-controlling interest	0	150	0	165
Net Income (Loss) Available to Common Stockholders, Basic	26,340	17,488	48,864	36,103
Segment Reporting Information, Additional Information [Abstract]
Total Assets	18,626,771	17,267,385	18,626,771	17,267,385	$ 17,792,274
On and Off Balance Sheet Program Assets, At Principal Balance	19,524,837	18,258,858	19,524,837	18,258,858
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	15,889	13,338	30,830	26,092
Less: reconciling adjustments(1)(2)(3)	(2,542)	(2,180)	(4,943)	(4,423)
Net Effective Spread	13,347	11,158	25,887	21,669
Guaranty Fee Income	4,488	4,191	8,867	8,486
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	341	994	899	1,188
Non-interest income	4,829	5,185	9,766	9,674
(Provision for)/release of loan losses	(424)	(327)	7	(964)
Provision for Other Credit Losses	(158)	(139)	(179)	54
Noninterest Expense, Excluding Loan, Lease and Other Losses	(4,954)	(4,446)	(9,474)	(8,511)
Non-interest expense	(5,112)	(4,585)	(9,653)	(8,457)
Segment core earnings before income taxes	12,640	11,431	26,007	21,922
Income Tax Expense (Benefit)	(2,654)	(4,001)	(5,461)	(7,673)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	9,986	7,430	20,546	14,249
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	9,986	7,430	20,546	14,249
Segment Reporting Information, Additional Information [Abstract]
Total Assets	4,428,172	3,958,344	4,428,172	3,958,344
On and Off Balance Sheet Program Assets, At Principal Balance	7,045,397	6,426,518	7,045,397	6,426,518
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	5,072	5,176	10,142	10,459
Less: reconciling adjustments(1)(2)(3)	(674)	(625)	(1,344)	(1,347)
Net Effective Spread	4,398	4,551	8,798	9,112
Guaranty Fee Income	190	99	356	173
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	8	11	13	25
Non-interest income	198	110	369	198
(Provision for)/release of loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(1,312)	(1,166)	(2,505)	(2,253)
Non-interest expense	(1,312)	(1,166)	(2,505)	(2,253)
Segment core earnings before income taxes	3,284	3,495	6,662	7,057
Income Tax Expense (Benefit)	(690)	(1,223)	(1,399)	(2,470)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,594	2,272	5,263	4,587
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	2,594	2,272	5,263	4,587
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,177,345	2,141,569	2,177,345	2,141,569
On and Off Balance Sheet Program Assets, At Principal Balance	2,418,115	2,237,013	2,418,115	2,237,013
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	3,313	3,003	5,850	5,951
Less: reconciling adjustments(1)(2)(3)	(390)	(334)	23	(714)
Net Effective Spread	2,923	2,669	5,873	5,237
Guaranty Fee Income	402	487	851	979
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	5	5	10	10
Non-interest income	407	492	861	989
(Provision for)/release of loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(739)	(643)	(1,412)	(1,230)
Non-interest expense	(739)	(643)	(1,412)	(1,230)
Segment core earnings before income taxes	2,591	2,518	5,322	4,996
Income Tax Expense (Benefit)	(544)	(881)	(1,118)	(1,748)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,047	1,637	4,204	3,248
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	2,047	1,637	4,204	3,248
Segment Reporting Information, Additional Information [Abstract]
Total Assets	995,068	1,038,383	995,068	1,038,383
On and Off Balance Sheet Program Assets, At Principal Balance	1,669,440	1,883,909	1,669,440	1,883,909
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	18,805	15,431	36,637	28,933
Less: reconciling adjustments(1)(2)(3)	(3,585)	(964)	(6,593)	(1,851)
Net Effective Spread	15,220	14,467	30,044	27,082
Guaranty Fee Income	91	165	180	620
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	0	0	0	0
Non-interest income	91	165	180	620
(Provision for)/release of loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(2,030)	(1,622)	(3,876)	(3,143)
Non-interest expense	(2,030)	(1,622)	(3,876)	(3,143)
Segment core earnings before income taxes	13,281	13,010	26,348	24,559
Income Tax Expense (Benefit)	(2,789)	(4,554)	(5,533)	(8,596)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	10,492	8,456	20,815	15,963
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	10,492	8,456	20,815	15,963
Segment Reporting Information, Additional Information [Abstract]
Total Assets	8,144,763	7,425,774	8,144,763	7,425,774
On and Off Balance Sheet Program Assets, At Principal Balance	8,391,885	7,711,418	8,391,885	7,711,418
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	854	2,783	3,703	5,367
Less: reconciling adjustments(1)(2)(3)	(580)	(294)	(1,042)	(607)
Net Effective Spread	274	2,489	2,661	4,760
Guaranty Fee Income	0	0	0	0
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	(209)	(146)	(349)	121
Non-interest income	(209)	(146)	(349)	121
(Provision for)/release of loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(3,728)	(3,374)	(7,117)	(6,856)
Non-interest expense	(3,728)	(3,374)	(7,117)	(6,856)
Segment core earnings before income taxes	(3,663)	(1,031)	(4,805)	(1,975)
Income Tax Expense (Benefit)	1,200	352	1,775	1,336
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	(2,463)	(679)	(3,030)	(639)
Preferred Stock Dividends, Income Statement Impact	(3,296)	(3,296)	(6,591)	(6,591)
Less: Net loss attributable to non-controlling interest		150		165
Segment core earnings	(5,759)	(3,825)	(9,621)	(7,065)
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,881,423	2,703,315	2,881,423	2,703,315
Segment Reconciling Items [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	0	0	0	0
Less: reconciling adjustments(1)(2)(3)	7,771	4,397	13,899	8,942
Net Effective Spread	7,771	4,397	13,899	8,942
Guaranty Fee Income	(1,690)	(1,470)	(3,274)	(2,942)
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	2,754	(592)	(934)	1,880
Non-interest income	1,064	(2,062)	(4,208)	(1,062)
(Provision for)/release of loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	0	0	0	0
Non-interest expense	0	0	0	0
Segment core earnings before income taxes	8,835	2,335	9,691	7,880
Income Tax Expense (Benefit)	(1,855)	(817)	(2,034)	(2,759)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	6,980	1,518	7,657	5,121
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	6,980	1,518	7,657	5,121
Segment Reporting Information, Additional Information [Abstract]
Total Assets	0	0	0	0
On and Off Balance Sheet Program Assets, At Principal Balance	$ 0	$ 0	$ 0	$ 0