Stock-Based Compensation (Components of Stock-Based Compensation Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity classified awards expense	$ 3.4	$ 6.2
Liability classified awards expense	0.5	0.2
Total pre-tax stock-based compensation expense	3.9	6.4
Book income tax benefit	(1.0)	(2.4)
Total stock-based compensation expense, net of tax	$ 2.9	$ 4.0