Stock-Based Compensation (Components of Stock-Based Compensation Expense) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity classified awards expense	$ 4.1	$ 3.4
Liability classified awards expense	1.0	0.5
Total pre-tax stock-based compensation expense	5.1	3.9
Book income tax benefit	(1.3)	(1.0)
Total stock-based compensation expense, net of tax	$ 3.8	$ 2.9