Fair Value Disclosures - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	$ 0.0	$ 0.1
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets and Liabilities, Fair Value Disclosure	$ 5,500.0	$ 4,900.0
Fair Value, Assets and Liabilities Fair Value as Percent of Total Assets	31.00%	31.00%
Fair Value, Assets and Liabilities, Fair Value as Percent of Total Financial Instruments Measured at Fair Value	71.00%	65.00%