Guarantees and Long Term Standby Purchase Commitments - Long-Term Standby Purchase Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 4,000,000	$ 4,900,000
Long-Term Standby Purchase Commitments [Member]
Guarantor Obligations [Line Items]
Weighted average remaining maturity, Loans underlying guarantees not held by transferor	15 years 3 months 10 days	15 years 1 month 10 days
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 3,100,000	$ 3,088,007
Long-Term Standby Purchase Commitments [Member] | Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	34,800	31,800
Farm & Ranch Loans and USDA Guarantees [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell	46,300	114,500
Rural Utilities Loans [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell	$ 8,000	$ 0