Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance	1,593,000
Nonvested shares subject to restriction, weighted average exercise price	$ 38.86
Restricted shares granted to employees	625,000
Granted shares subject to restriction, weighted average exercise price	$ 52.64
Shares subject to restriction, Vested	(486,000)
Vested shares subject to restriction, weighted average exercise price	$ 41.05
Shares subject to restriction, Forfeited	(7,000)
Forfeited shares subject to restriction, weighted average exercise price	$ 49.48
Nonvested shares subject to restriction, Ending Balance	1,725,000
Nonvested shares subject to restriction, weighted average exercise price	$ 43.26
Restricted Stock [Member]
Restricted stock compensation expense
Restricted stock compensation expense	$ 6.5	$ 6.8
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 50.6
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 3 months 18 days
Status of nonvested restricted stock
Restricted shares granted to employees	625,416
Performance Shares [Member]
Status of nonvested restricted stock
Performance share compensation expense	$ 1.7	3.1
Relative Revenue Growth Performance Share Plans [Member]
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance	355,000
Nonvested shares subject to restriction, weighted average exercise price	$ 39.42
Restricted shares granted to employees	175,000
Granted shares subject to restriction, weighted average exercise price	$ 52.64
Nonvested shares subject to restriction, Ending Balance	530,000
Nonvested shares subject to restriction, weighted average exercise price	$ 43.79
Performance share compensation expense	$ 6.5	$ 3.2
2014 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	8,000,000
Number of shares available for grant	4,400,000
First Half Vested | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of awards vested	50.00%
Vesting period	2 years
Second Half Vested | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years