Share-Based Compensation - Compensation Expense for Restricted Stock and Performance Units (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 5.6	$ 4.7	$ 10.2	$ 10.3
Income tax benefit	(2.1)	(1.8)	(3.9)	(4.0)
Share-based compensation expense, net of tax benefit	3.5	2.9	6.3	6.3
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	4.1	3.9	7.5	8.8
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 1.5	$ 0.8	$ 2.7	$ 1.5