Share-Based Compensation - Compensation Expense for Share-Based Awards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 19.7	$ 18.2	$ 15.6
Income tax benefit	(7.7)	(7.1)	(6.1)
Impact on net income	12.0	11.1	9.5
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	15.8	15.2	13.8
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 3.9	$ 3.0	$ 1.8