Stock-Based Compensation - Performance Share Plans (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum	200.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms	2 years
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Nonvested shares, number, beginning balance	409,600	474,600
Nonvested shares, weighted average price, beginning balance	$ 43.99	$ 56.55
Nonvested shares, number, ending balance	356,750	409,600	474,600
Nonvested shares, weighted average price, ending balance	$ 32.26	$ 43.99	$ 56.55
Performance share plan expense	$ 9.9	$ 9.6	$ 12.2
Performance share plan, shares issued in period	0	0	0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 7.2
Relative Revenue Growth Performance Share Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum	200.00%
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms	2 years
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Nonvested shares, number, beginning balance	320,000	0
Nonvested shares, weighted average price, beginning balance	$ 38.62	$ 0.00
Nonvested shares, granted	198,000	485,000
Nonvested shares, weighted average price, granted	$ 40.08	$ 38.62
Nonvested shares, vested	(156,000)	(126,000)
Nonvested shares, weighted average price, vested	$ 38.62	$ 38.62
Nonvested shares, forfeited	(7,000)	(39,000)
Nonvested shares, weighted average price, forfeited	$ 39.20	$ 38.62
Nonvested shares, number, ending balance	355,000	320,000	0
Nonvested shares, weighted average price, ending balance	$ 39.42	$ 38.62	$ 0.00
Performance share plan expense	$ 15.9	$ 7.1
Performance share plan, shares issued in period	126,000	0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 9.3