Guarantees and Long Term Standby Purchase Commitments - Long-Term Standby Purchase Commitments (Details) - Long-Term Standby Purchase Commitments [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Guarantor Obligations [Line Items]
Weighted average remaining maturity, Loans underlying guarantees not held by transferor	15 years 5 months 20 days	15 years 3 months 10 days
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 3,000.0		$ 3,100.0
Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 35.2		$ 34.8