Guarantees and Long Term Standby Purchase Commitments - Long-Term Standby Purchase Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 4,000.0	$ 4,000.0
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 20 days	15 years 3 months 10 days
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 3,200.0	$ 3,100.0
Long-Term Standby Purchase Commitments [Member] | Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	35.9	34.8
Farm & Ranch Loans and USDA Guarantees [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell	$ 37.1	46.3
Rural Utilities Loans [Member]
Guarantor Obligations [Line Items]
Loans and Leases Receivable, Commitments to Purchase or Sell		$ 8.0