Share-Based Compensation - Compensation Expense for Restricted Stock and Performance Units (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 20.6	$ 19.7	$ 18.2
Income tax benefit	(7.9)	(7.7)	(7.1)
Impact on net income	12.7	12.0	11.1
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	15.0	15.8	15.2
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 5.6	$ 3.9	$ 3.0