Business Segment Reporting - Core Earnings - Business Segments (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments	4
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	$ 43,229	$ 37,071	$ 125,807
Less: reconciling adjustments(1)(2)(3)	0	0	0
Net Effective Spread	0	0	0
Guaranty Fee Income	3,499	3,844	14,077
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	(3,260)	2,952	6,064
Non-interest income	239	6,796	20,141
Release of/(provision for) loan losses	431	(637)	(2,388)
Provision for Other Credit Losses	(21)	193	2,180
Noninterest Expense, Excluding Loan, Lease and Other Losses	(11,621)	(10,742)	(37,662)
Non-interest expense	(11,642)	(10,549)	(35,482)
Income Tax Expense (Benefit)	(6,438)	(10,786)	(34,239)
Net income	25,819	21,895	73,839
Net income	32,257	32,681	108,078
Preferred Stock Dividends, Income Statement Impact	(3,295)	(3,295)	(13,182)
Less: Net loss attributable to non-controlling interest	0	15	(5,139)
Preferred Stock Redemption Premium			8,147
Net Income (Loss) Available to Common Stockholders, Basic	22,524	18,615	47,371
Segment Reporting Information, Additional Information [Abstract]
Total Assets	18,397,236	15,931,757	15,540,354	$ 17,792,274
On and Off Balance Sheet Program Assets, At Principal Balance	19,379,443	17,844,541	15,898,820
Farm & Ranch [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	14,941	12,754	43,271
Less: reconciling adjustments(1)(2)(3)	(2,401)	(2,243)	(5,308)
Net Effective Spread	12,540	10,511	37,963
Guaranty Fee Income	4,378	4,295	15,076
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	557	194	1,039
Non-interest income	4,935	4,489	16,115
Release of/(provision for) loan losses	431	(637)	(2,388)
Provision for Other Credit Losses	(21)	193	2,180
Noninterest Expense, Excluding Loan, Lease and Other Losses	(4,520)	(4,065)	(16,876)
Non-interest expense	(4,541)	(3,872)	(14,696)
Segment core earnings before income taxes	13,365	10,491	36,994
Income Tax Expense (Benefit)	(2,807)	(3,672)	(12,956)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	10,558	6,819	24,038
Preferred Stock Dividends, Income Statement Impact	0	0	0
Less: Net loss attributable to non-controlling interest		0	0
Preferred Stock Redemption Premium			0
Segment core earnings	10,558	6,819	24,038
Segment Reporting Information, Additional Information [Abstract]
Total Assets	4,306,960	3,693,360	3,041,386
On and Off Balance Sheet Program Assets, At Principal Balance	6,932,002	6,240,467	5,725,299
USDA Guarantees [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	5,070	5,283	17,750
Less: reconciling adjustments(1)(2)(3)	(670)	(722)	(638)
Net Effective Spread	4,400	4,561	17,112
Guaranty Fee Income	166	74	17
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	5	14	100
Non-interest income	171	88	117
Release of/(provision for) loan losses	0	0	0
Provision for Other Credit Losses	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(1,193)	(1,087)	(3,449)
Non-interest expense	(1,193)	(1,087)	(3,449)
Segment core earnings before income taxes	3,378	3,562	13,780
Income Tax Expense (Benefit)	(709)	(1,247)	(4,826)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,669	2,315	8,954
Preferred Stock Dividends, Income Statement Impact	0	0	0
Less: Net loss attributable to non-controlling interest		0	0
Preferred Stock Redemption Premium			0
Segment core earnings	2,669	2,315	8,954
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,195,714	2,109,264	1,977,609
On and Off Balance Sheet Program Assets, At Principal Balance	2,391,739	2,149,697	1,918,277
Rural Utilities [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,537	2,948	11,728
Less: reconciling adjustments(1)(2)(3)	413	(380)	(723)
Net Effective Spread	2,950	2,568	11,005
Guaranty Fee Income	449	492	397
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	5	5	25
Non-interest income	454	497	422
Release of/(provision for) loan losses	0	0	0
Provision for Other Credit Losses	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(673)	(587)	(3,364)
Non-interest expense	(673)	(587)	(3,364)
Segment core earnings before income taxes	2,731	2,478	8,063
Income Tax Expense (Benefit)	(574)	(867)	(2,828)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	2,157	1,611	5,235
Preferred Stock Dividends, Income Statement Impact	0	0	0
Less: Net loss attributable to non-controlling interest		0	0
Preferred Stock Redemption Premium			0
Segment core earnings	2,157	1,611	5,235
Segment Reporting Information, Additional Information [Abstract]
Total Assets	1,043,335	1,005,187	1,019,279
On and Off Balance Sheet Program Assets, At Principal Balance	1,729,797	1,868,794	1,530,990
Institutional Credit [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	17,832	13,502	44,971
Less: reconciling adjustments(1)(2)(3)	(3,008)	(887)	(1,512)
Net Effective Spread	14,824	12,615	43,459
Guaranty Fee Income	90	455	1,665
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	0	0	0
Non-interest income	90	455	1,665
Release of/(provision for) loan losses	0	0	0
Provision for Other Credit Losses	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(1,846)	(1,521)	(2,109)
Non-interest expense	(1,846)	(1,521)	(2,109)
Segment core earnings before income taxes	13,068	11,549	43,015
Income Tax Expense (Benefit)	(2,744)	(4,042)	(15,038)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	10,324	7,507	27,977
Preferred Stock Dividends, Income Statement Impact	0	0	0
Less: Net loss attributable to non-controlling interest		0	0
Preferred Stock Redemption Premium			0
Segment core earnings	10,324	7,507	27,977
Segment Reporting Information, Additional Information [Abstract]
Total Assets	8,066,231	6,315,591	5,420,195
On and Off Balance Sheet Program Assets, At Principal Balance	8,325,905	7,585,583	6,724,254
Corporate Segment [Member]
Segment Reporting Information, Profit (Loss) [Abstract]
Net interest income	2,849	2,584	8,087
Less: reconciling adjustments(1)(2)(3)	(462)	(313)	(198)
Net Effective Spread	2,387	2,271	7,889
Guaranty Fee Income	0	0	0
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	(139)	267	74
Non-interest income	(139)	267	74
Release of/(provision for) loan losses	0	0	0
Provision for Other Credit Losses	0	0	0
Noninterest Expense, Excluding Loan, Lease and Other Losses	(3,389)	(3,482)	(11,864)
Non-interest expense	(3,389)	(3,482)	(11,864)
Segment core earnings before income taxes	(1,141)	(944)	(3,901)
Income Tax Expense (Benefit)	575	984	3,053
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	(566)	40	(848)
Preferred Stock Dividends, Income Statement Impact	(3,295)	(3,295)	(13,182)
Less: Net loss attributable to non-controlling interest		15	(5,139)
Preferred Stock Redemption Premium			0
Segment core earnings	(3,861)	(3,240)	(19,169)
Segment Reporting Information, Additional Information [Abstract]
Total Assets	2,784,996	2,808,355	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)	6,128	4,545	8,379
Net Effective Spread	6,128	4,545	8,379
Guaranty Fee Income	(1,584)	(1,472)	(3,078)
Other Non-Interest Income (Expense), Excluding Guarantee and Commitment Fees	(3,688)	2,472	4,826
Non-interest income	(5,272)	1,000	1,748
Release of/(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	856	5,545	10,127
Income Tax Expense (Benefit)	(179)	(1,942)	(1,644)
Segment core earnings before preferred stock dividends and attibution of income to non-controlling interest - preferred stock dividends	677	3,603	8,483
Preferred Stock Dividends, Income Statement Impact	0	0	0
Less: Net loss attributable to non-controlling interest		0	0
Preferred Stock Redemption Premium			8,147
Segment core earnings	677	3,603	336
Segment Reporting Information, Additional Information [Abstract]
Total Assets	0	0	0
On and Off Balance Sheet Program Assets, At Principal Balance	$ 0	$ 0	$ 0