Guarantees and Long Term Standby Purchase Commitments - Off-Balance Sheet Guaranteed Securities (Details) - Variable Interest Entity, Not Primary Beneficiary [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 910,488	$ 899,284
Farm & Ranch [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	314,497	333,511
USDA Guarantees [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	284,435	254,217
Institutional Credit [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	11,556	11,556
Revolving Credit Facility [Member] | Institutional Credit [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 300,000	$ 300,000