Derivative Instruments (Net Cash Received (Paid) Related to Derivatives) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Total operating activities	$ 244.4	$ 6.8	$ 4.3
Total investing activities	0.0	(0.5)	(2.9)
Total financing activities	112.8	(138.1)	(251.5)
Total	$ 357.2	$ (131.8)	$ (250.1)