Organization - Organization (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Product Information [Line Items]
On and Off Balance Sheet Program Assets, At Principal Balance	$ 19,007,311	$ 17,399,475	$ 15,898,820
Farm & Ranch [Member]
Product Information [Line Items]
On and Off Balance Sheet Program Assets, At Principal Balance	6,867,586	6,139,304	5,725,299
USDA Guarantees [Member]
Product Information [Line Items]
On and Off Balance Sheet Program Assets, At Principal Balance	2,352,214	2,094,375	1,918,277
Rural Utilities [Member]
Product Information [Line Items]
On and Off Balance Sheet Program Assets, At Principal Balance	1,882,633	1,878,110	1,530,990
Institutional Credit [Member]
Product Information [Line Items]
On and Off Balance Sheet Program Assets, At Principal Balance	7,904,878	$ 7,287,686	$ 6,724,254
Notes Payable to Banks [Member]
Product Information [Line Items]
Notes Payable	1,700,000
Medium-term Notes [Member]
Product Information [Line Items]
Notes Payable	$ 13,800,000