Derivative Financial Instruments and Credit Risk - Derivative Assets at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross Amounts of Recognized Assets	$ 5.2	$ 7.2
Gross Amounts Offset in the Consolidated Balance Sheets	0.0	0.0
Amount Presented in the Consolidated Balance Sheets	5.2	7.2
Amount Subject to Master Netting Agreement	(1.3)	(4.3)
Net Amount	$ 3.9	$ 2.9