Derivative Financial Instruments and Risk Management - Derivative Assets at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Assets	$ 11.0	$ 5.2
Gross Amounts Offset in the Consolidated Balance Sheets	0.0	0.0
Net Presented in the Consolidated Balance Sheets	11.0	5.2
Amount Subject to Master Netting Agreement	(4.0)	(1.3)
Net Amount	$ 7.0	$ 3.9