Share-based Compensation - Compensation Expense for Restricted Stock and Performance Units (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 4.8	$ 4.6	$ 15.0	$ 13.7
Income tax benefit	(1.8)	(1.8)	(5.8)	(5.3)
Share-based compensation expense, net of tax benefit	3.0	2.8	9.2	8.4
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	3.5	3.7	12.2	11.7
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total share-based compensation expense	$ 1.3	$ 0.9	$ 2.8	$ 2.0