Business Segment Reporting - Core Earnings - Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 45,058	$ 39,562	$ 132,220	$ 116,364
Less: reconciling adjustments(1)(2)(3)	0	0	0	0
Net Effective Spread	0	0	0	0
Guaranty Fee Income	3,490	3,314	10,470	10,630
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	949	985	588	4,209
Non-interest income	4,439	4,299	11,058	14,839
Provision for loan losses	(99)	(270)	(92)	(1,234)
Provision for Other Credit Losses	102	(114)	(77)	(60)
Noninterest Expense, Excluding Loan, Lease and Other Losses	(11,752)	(10,502)	(36,136)	(32,495)
Non-interest expense	(11,650)	(10,616)	(36,213)	(32,555)
Income Tax Expense (Benefit)	(7,979)	(11,193)	(21,749)	(33,103)
Net income	29,769	21,782	85,224	64,311
Net income	37,748	32,975	106,973	97,414
Preferred Stock Dividends, Income Statement Impact	(3,295)	(3,295)	(9,886)	(9,886)
Less: Net loss attributable to non-controlling interest	0	0	0	165
Net Income (Loss) Available to Common Stockholders, Basic	26,474	18,487	75,338	54,590
Segment Reporting Information, Additional Information [Abstract]
Total Assets	18,474,034	17,690,238	18,474,034	17,690,238	$ 17,792,274
On and Off Balance Sheet Program Assets, At Principal Balance	19,540,586	18,644,273	19,540,586	18,644,273
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	16,425	13,609	47,255	39,701
Less: reconciling adjustments(1)(2)(3)	(2,538)	(2,306)	(7,481)	(6,729)
Net Effective Spread	13,887	11,303	39,774	32,972
Guaranty Fee Income	4,489	4,236	13,356	12,722
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	294	214	1,193	1,402
Non-interest income	4,783	4,450	14,549	14,124
Provision for loan losses	(99)	(270)	(92)	(1,234)
Provision for Other Credit Losses	102	(114)	(77)	(60)
Noninterest Expense, Excluding Loan, Lease and Other Losses	(4,456)	(4,077)	(13,930)	(12,588)
Non-interest expense	(4,354)	(4,191)	(14,007)	(12,648)
Segment core earnings before income taxes	14,217	11,292	40,224	33,214
Income Tax Expense (Benefit)	(2,986)	(3,952)	(8,447)	(11,625)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	11,231	7,340	31,777	21,589
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	11,231	7,340	31,777	21,589
Segment Reporting Information, Additional Information [Abstract]
Total Assets	4,438,128	4,128,778	4,438,128	4,128,778
On and Off Balance Sheet Program Assets, At Principal Balance	7,072,018	6,557,030	7,072,018	6,557,030
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	5,304	5,288	15,446	15,747
Less: reconciling adjustments(1)(2)(3)	(677)	(560)	(2,021)	(1,907)
Net Effective Spread	4,627	4,728	13,425	13,840
Guaranty Fee Income	214	130	570	303
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	5	9	18	34
Non-interest income	219	139	588	337
Provision for loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(1,288)	(1,080)	(3,793)	(3,333)
Non-interest expense	(1,288)	(1,080)	(3,793)	(3,333)
Segment core earnings before income taxes	3,558	3,787	10,220	10,844
Income Tax Expense (Benefit)	(747)	(1,325)	(2,146)	(3,795)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,811	2,462	8,074	7,049
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	2,811	2,462	8,074	7,049
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,212,515	2,165,749	2,212,515	2,165,749
On and Off Balance Sheet Program Assets, At Principal Balance	2,471,251	2,298,956	2,471,251	2,298,956
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	3,081	2,230	8,931	8,181
Less: reconciling adjustments(1)(2)(3)	(204)	535	(181)	(179)
Net Effective Spread	2,877	2,765	8,750	8,002
Guaranty Fee Income	376	476	1,227	1,455
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	15	5	25	15
Non-interest income	391	481	1,252	1,470
Provision for loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(732)	(608)	(2,144)	(1,838)
Non-interest expense	(732)	(608)	(2,144)	(1,838)
Segment core earnings before income taxes	2,536	2,638	7,858	7,634
Income Tax Expense (Benefit)	(533)	(923)	(1,651)	(2,671)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,003	1,715	6,207	4,963
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	2,003	1,715	6,207	4,963
Segment Reporting Information, Additional Information [Abstract]
Total Assets	956,204	1,073,525	956,204	1,073,525
On and Off Balance Sheet Program Assets, At Principal Balance	1,632,037	1,886,445	1,632,037	1,886,445
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	17,600	15,431	54,237	44,364
Less: reconciling adjustments(1)(2)(3)	(1,958)	(976)	(8,551)	(2,827)
Net Effective Spread	15,642	14,455	45,686	41,537
Guaranty Fee Income	91	93	271	713
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	0	0	0	0
Non-interest income	91	93	271	713
Provision for loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(1,844)	(1,670)	(5,720)	(4,813)
Non-interest expense	(1,844)	(1,670)	(5,720)	(4,813)
Segment core earnings before income taxes	13,889	12,878	40,237	37,437
Income Tax Expense (Benefit)	(2,917)	(4,507)	(8,450)	(13,103)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	10,972	8,371	31,787	24,334
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	10,972	8,371	31,787	24,334
Segment Reporting Information, Additional Information [Abstract]
Total Assets	8,103,181	7,612,572	8,103,181	7,612,572
On and Off Balance Sheet Program Assets, At Principal Balance	8,365,280	7,901,842	8,365,280	7,901,842
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,648	3,004	6,351	8,371
Less: reconciling adjustments(1)(2)(3)	(604)	(279)	(1,646)	(886)
Net Effective Spread	2,044	2,725	4,705	7,485
Guaranty Fee Income	0	0	0	0
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	(245)	78	(594)	199
Non-interest income	(245)	78	(594)	199
Provision for loan losses	0	0	0	0
Provision for Other Credit Losses	0	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(3,432)	(3,067)	(10,549)	(9,923)
Non-interest expense	(3,432)	(3,067)	(10,549)	(9,923)
Segment core earnings before income taxes	(1,633)	(264)	(6,438)	(2,239)
Income Tax Expense (Benefit)	292	439	2,067	1,775
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	(1,341)	175	(4,371)	(464)
Preferred Stock Dividends, Income Statement Impact	(3,295)	(3,295)	(9,886)	(9,886)
Less: Net loss attributable to non-controlling interest		0		165
Segment core earnings	(4,636)	(3,120)	(14,257)	(10,185)
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,764,006	2,709,614	2,764,006	2,709,614
Segment Reconciling Items [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	0	0	0	0
Less: reconciling adjustments(1)(2)(3)	5,981	3,586	19,880	12,528
Net Effective Spread	5,981	3,586	19,880	12,528
Guaranty Fee Income	(1,680)	(1,621)	(4,954)	(4,563)
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	880	679	(54)	2,559
Non-interest income	(800)	(942)	(5,008)	(2,004)
Provision for 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	5,181	2,644	14,872	10,524
Income Tax Expense (Benefit)	(1,088)	(925)	(3,122)	(3,684)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	4,093	1,719	11,750	6,840
Preferred Stock Dividends, Income Statement Impact	0	0	0	0
Less: Net loss attributable to non-controlling interest		0		0
Segment core earnings	4,093	1,719	11,750	6,840
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