Share-Based Compensation - Compensation Expense for Restricted Stock and Performance Units (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 5.6	$ 5.6	$ 10.7	$ 10.2
Income tax benefit	(1.4)	(2.1)	(2.7)	(3.9)
Share-based compensation expense, net of tax benefit	4.2	3.5	8.0	6.3
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	5.0	4.1	8.9	7.5
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 0.6	$ 1.5	$ 1.8	$ 2.7