Stock-Based Compensation (Components of Stock-Based Compensation Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity classified awards expense	$ 2.3	$ 1.5	$ 5.7	$ 7.7
Liability classified awards expense (credit)	0.4	(1.3)	0.9	(1.1)
Total pre-tax stock-based compensation expense	2.7	0.2	6.6	6.6
Book income tax benefit	(0.7)	(0.1)	(1.7)	(2.5)
Total stock-based compensation expense, net of tax	$ 2.0	$ 0.1	$ 4.9	$ 4.1