Business Segment Reporting - Core Earnings - Business Segments (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments									4
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 41,283	$ 39,562	$ 39,731	$ 37,071	$ 36,713	$ 35,563	$ 34,358	$ 33,640	$ 157,647	$ 140,274	$ 125,807
Less: reconciling adjustments(1)(2)(3)									0	0	0
Net Effective Spread									0	0	0
Guaranty Fee Income	3,484	3,314	3,472	3,844	3,789	3,798	3,655	3,626	14,114	14,868	14,077
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									3,398	5,600	6,064
Non-interest income	2,673	4,299	3,744	6,796	19,307	4,101	(234)	(2,706)	17,512	20,468	20,141
Provision for loan losses	(474)	(270)	(327)	(637)	(461)	(191)	(364)	(49)	(1,708)	(1,065)	(2,388)
Provision for Other Credit Losses									(50)	63	2,180
Noninterest Expense, Excluding Loan, Lease and Other Losses									(42,715)	(40,383)	(37,662)
Non-interest expense	(10,210)	(10,616)	(11,390)	(10,549)	(10,977)	(9,303)	(10,074)	(9,966)	(42,765)	(40,320)	(35,482)
Income Tax Expense (Benefit)	(13,266)	(11,193)	(11,124)	(10,786)	(15,793)	(10,529)	(8,400)	(7,335)	(46,369)	(42,057)	(34,239)
Net income	20,006	21,782	20,634	21,895	28,789	19,641	15,286	13,584	84,317	77,300	73,839
Net income	33,272	32,975	31,758	32,681	44,582	30,170	23,686	20,919	130,686	119,357	108,078
Preferred Stock Dividends, Income Statement Impact	(3,296)	(3,295)	(3,296)	(3,295)	(3,296)	(3,295)	(3,296)	(3,295)	(13,182)	(13,182)	(13,182)
Less: Net loss/(income) attributable to non-controlling interest	0	0	150	15	(28)	18	16	28	165	34	(5,139)
Preferred Stock Redemption Premium									0	0	8,147
Net Income (Loss) Available to Common Stockholders, Basic	16,710	$ 18,487	$ 17,488	$ 18,615	25,465	$ 16,364	$ 12,006	$ 10,317	71,300	64,152	47,371
Segment Reporting Information, Additional Information [Abstract]
Total Assets	17,792,274				15,606,020				17,792,274	15,606,020	15,540,354
On and Off Balance Sheet Program Assets, At Principal Balance	19,007,311				17,399,475				19,007,311	17,399,475	15,898,820
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									54,290	47,219	43,271
Less: reconciling adjustments(1)(2)(3)									(8,922)	(7,729)	(5,308)
Net Effective Spread									45,368	39,490	37,963
Guaranty Fee Income									17,175	15,542	15,076
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									2,449	539	1,039
Non-interest income									19,624	16,081	16,115
Provision for loan losses									(1,708)	(1,065)	(2,388)
Provision for Other Credit Losses									(50)	63	2,180
Noninterest Expense, Excluding Loan, Lease and Other Losses									(16,554)	(16,206)	(16,876)
Non-interest expense									(16,604)	(16,143)	(14,696)
Segment core earnings before income taxes									46,680	38,363	36,994
Income Tax Expense (Benefit)									(16,338)	(13,428)	(12,956)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									30,342	24,935	24,038
Preferred Stock Dividends, Income Statement Impact									0	0	0
Less: Net loss/(income) attributable to non-controlling interest									0	0	0
Preferred Stock Redemption Premium											0
Segment core earnings									30,342	24,935	24,038
Segment Reporting Information, Additional Information [Abstract]
Total Assets	4,274,693				3,582,098				4,274,693	3,582,098	3,041,386
On and Off Balance Sheet Program Assets, At Principal Balance	6,867,586				6,139,304				6,867,586	6,139,304	5,725,299
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									21,106	21,865	17,750
Less: reconciling adjustments(1)(2)(3)									(2,287)	(3,210)	(638)
Net Effective Spread									18,819	18,655	17,112
Guaranty Fee Income									456	101	17
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									43	222	100
Non-interest income									499	323	117
Provision for loan losses									0	0	0
Provision for Other Credit Losses									0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses									(4,384)	(4,200)	(3,449)
Non-interest expense									(4,384)	(4,200)	(3,449)
Segment core earnings before income taxes									14,934	14,778	13,780
Income Tax Expense (Benefit)									(5,227)	(5,173)	(4,826)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									9,707	9,605	8,954
Preferred Stock Dividends, Income Statement Impact									0	0	0
Less: Net loss/(income) attributable to non-controlling interest									0	0	0
Preferred Stock Redemption Premium											0
Segment core earnings									9,707	9,605	8,954
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,195,189				2,096,503				2,195,189	2,096,503	1,977,609
On and Off Balance Sheet Program Assets, At Principal Balance	2,352,214				2,094,375				2,352,214	2,094,375	1,918,277
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									11,598	11,739	11,728
Less: reconciling adjustments(1)(2)(3)									(539)	(1,771)	(723)
Net Effective Spread									11,059	9,968	11,005
Guaranty Fee Income									1,914	1,694	397
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									20	2	25
Non-interest income									1,934	1,696	422
Provision for loan losses									0	0	0
Provision for Other Credit Losses									0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses									(2,430)	(2,856)	(3,364)
Non-interest expense									(2,430)	(2,856)	(3,364)
Segment core earnings before income taxes									10,563	8,808	8,063
Income Tax Expense (Benefit)									(3,696)	(3,083)	(2,828)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									6,867	5,725	5,235
Preferred Stock Dividends, Income Statement Impact									0	0	0
Less: Net loss/(income) attributable to non-controlling interest									0	0	0
Preferred Stock Redemption Premium											0
Segment core earnings									6,867	5,725	5,235
Segment Reporting Information, Additional Information [Abstract]
Total Assets	1,088,986				1,012,014				1,088,986	1,012,014	1,019,279
On and Off Balance Sheet Program Assets, At Principal Balance	1,882,633				1,878,110				1,882,633	1,878,110	1,530,990
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									59,842	48,756	44,971
Less: reconciling adjustments(1)(2)(3)									(3,505)	(3,184)	(1,512)
Net Effective Spread									56,337	45,572	43,459
Guaranty Fee Income									805	1,833	1,665
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									0	0	0
Non-interest income									805	1,833	1,665
Provision for loan losses									0	0	0
Provision for Other Credit Losses									0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses									(6,439)	(3,786)	(2,109)
Non-interest expense									(6,439)	(3,786)	(2,109)
Segment core earnings before income taxes									50,703	43,619	43,015
Income Tax Expense (Benefit)									(17,746)	(15,265)	(15,038)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									32,957	28,354	27,977
Preferred Stock Dividends, Income Statement Impact									0	0	0
Less: Net loss/(income) attributable to non-controlling interest									0	0	0
Preferred Stock Redemption Premium											0
Segment core earnings									32,957	28,354	27,977
Segment Reporting Information, Additional Information [Abstract]
Total Assets	7,627,749				6,008,574				7,627,749	6,008,574	5,420,195
On and Off Balance Sheet Program Assets, At Principal Balance	7,904,878				7,287,686				7,904,878	7,287,686	6,724,254
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									10,811	10,695	8,087
Less: reconciling adjustments(1)(2)(3)									(1,091)	(1,308)	(198)
Net Effective Spread									9,720	9,387	7,889
Guaranty Fee Income									0	0	0
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									171	1,322	74
Non-interest income									171	1,322	74
Provision for loan losses									0	0	0
Provision for Other Credit Losses									0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses									(12,908)	(13,335)	(11,864)
Non-interest expense									(12,908)	(13,335)	(11,864)
Segment core earnings before income taxes									(3,017)	(2,626)	(3,901)
Income Tax Expense (Benefit)									1,792	636	3,053
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									(1,225)	(1,990)	(848)
Preferred Stock Dividends, Income Statement Impact									(13,182)	(13,182)	(13,182)
Less: Net loss/(income) attributable to non-controlling interest									165	34	(5,139)
Preferred Stock Redemption Premium											0
Segment core earnings									(14,242)	(15,138)	(19,169)
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,605,657				2,906,831				2,605,657	2,906,831	4,081,885
Segment Reconciling Items [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income									0	0	0
Less: reconciling adjustments(1)(2)(3)									16,344	17,202	8,379
Net Effective Spread									16,344	17,202	8,379
Guaranty Fee Income									(6,236)	(4,302)	(3,078)
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees									715	3,515	4,826
Non-interest income									(5,521)	(787)	1,748
Provision for loan losses									0	0	0
Provision for Other Credit Losses									0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses									0	0	0
Non-interest expense									0	0	0
Segment core earnings before income taxes									10,823	16,415	10,127
Income Tax Expense (Benefit)									(5,154)	(5,744)	(1,644)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends									5,669	10,671	8,483
Preferred Stock Dividends, Income Statement Impact									0	0	0
Less: Net loss/(income) attributable to non-controlling interest									0	0	0
Preferred Stock Redemption Premium											8,147
Segment core earnings									5,669	10,671	336
Segment Reporting Information, Additional Information [Abstract]
Total Assets	0				0				0	0	0
On and Off Balance Sheet Program Assets, At Principal Balance	$ 0				$ 0				$ 0	$ 0	$ 0