Share-Based Compensation - Compensation Expense for Share-Based Awards (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 15.6	$ 14.8	$ 11.7
Income tax benefit	(6.1)	(5.8)	(4.6)
Impact on net income	9.5	9.0	7.1
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	13.8	14.3	11.7
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 1.8	$ 0.5	$ 0