Guarantees and Long-Term Standby Purchase Commitments - Long-Term Standby Purchase Commitments (Details) - Long-Term Standby Purchase Commitments [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Maximum principal amount of potential undiscounted future payments	$ 3,100.0	$ 3,200.0
Weighted average remaining maturity, loans underlying guarantees not held by transferor	15 years 4 months 20 days	15 years 3 months 20 days
Guarantee obligations issued after January 1, 2003 [Member]
Guarantor Obligations [Line Items]
Guarantor obligation, liability	$ 35.6	$ 35.9