Stock-Based Compensation (Components of Stock-Based Compensation Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity classified awards expense	$ 6.2	$ 3.2
Liability classified awards expense	0.2	0.8
Total pre-tax stock-based compensation expense	6.4	4.0
Book income tax benefit	(2.4)	(1.5)
Total stock-based compensation expense, net of tax	$ 4.0	$ 2.5