Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				5 Months Ended	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Feb. 28, 2019	Sep. 30, 2018	Sep. 30, 2017
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance					1,556,000
Nonvested shares subject to restriction, weighted average exercise price					$ 43.03
Shares subject to restriction, Forfeited	(27,000)
Forfeited shares subject to restriction, weighted average exercise price	$ 43.09
Nonvested shares subject to restriction, Ending Balance	1,556,000					1,556,000
Nonvested shares subject to restriction, weighted average exercise price	$ 43.03					$ 43.03
Net impact due to stock awards modification	$ 5.9					$ 1.8
Restricted Stock [Member]
Restricted stock compensation expense
Restricted stock compensation expense	6.8			$ 6.7		18.7	$ 20.2
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 36.4					$ 36.4
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition						1 year 9 months
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance	1,583,000	1,725,000	1,593,000			1,593,000
Nonvested shares subject to restriction, weighted average exercise price	$ 43.03	$ 43.26	$ 38.86			$ 38.86
Restricted shares granted to employees		68,000	625,000			673,961
Granted shares subject to restriction, weighted average exercise price		$ 50.86	$ 52.64
Shares subject to restriction, Vested		(68,000)	(486,000)
Vested shares subject to restriction, weighted average exercise price		$ 51.11	$ 41.05
Shares subject to restriction, Forfeited		(142,000)	(7,000)
Forfeited shares subject to restriction, weighted average exercise price		$ 45.68	$ 49.48
Nonvested shares subject to restriction, Ending Balance		1,583,000	1,725,000
Nonvested shares subject to restriction, weighted average exercise price		$ 43.03	$ 43.26
Performance Shares [Member]
Status of nonvested restricted stock
Performance share compensation expense	$ 3.4			2.2		$ 5.6	7.6
Relative Revenue Growth Performance Share Plans [Member]
Status of nonvested restricted stock
Nonvested shares subject to restriction, Beginning Balance	481,000	530,000	355,000		479,000	355,000
Nonvested shares subject to restriction, weighted average exercise price	$ 44.45	$ 43.79	$ 39.42		$ 44.43	$ 39.42
Restricted shares granted to employees		95,000	175,000
Granted shares subject to restriction, weighted average exercise price		$ 50.20	$ 52.64
Shares subject to restriction, Forfeited	(2,000)	(144,000)
Forfeited shares subject to restriction, weighted average exercise price	$ 47.43	$ 45.82
Nonvested shares subject to restriction, Ending Balance	479,000	481,000	530,000			479,000
Nonvested shares subject to restriction, weighted average exercise price	$ 44.43	$ 44.45	$ 43.79			$ 44.43
Performance share compensation expense	$ 5.6			$ 2.3		$ 13.3	$ 7.7
2018 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	7,000,000					7,000,000
Number of shares available for grant	6,900,000					6,900,000
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
Director [Member] | Restricted Stock [Member]
Status of nonvested restricted stock
Restricted shares granted to employees						19,656
Scenario, Forecast [Member]
Status of nonvested restricted stock
Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Incremental Compensation Cost					$ 9.9