Stock-Based Compensation - Performance Share Plans (Details) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	12 Months Ended
	Feb. 28, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Plan Modification, Incremental Compensation Cost		$ 8.3	$ 0.0	$ 0.0
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%
Performance share plan expense		$ 9.0	$ 9.9	$ 9.6
Performance share plan, shares issued in period		0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Nonvested shares, number, beginning balance	297,000,000	355,000,000	410,000,000	475,000,000
Nonvested shares, weighted average price, beginning balance	$ 60.35	$ 32.35	$ 43.99	$ 56.55
Nonvested shares, granted		287,000,000	201,000,000	171,000,000
Nonvested shares, weighted average price, granted		$ 68.38	$ 45.57	$ 16.61
Nonvested shares, forfeited		(345,000,000)	(256,000,000)	(236,000,000)
Nonvested shares, weighted average price, forfeited		$ 38.26	$ 61.40	$ 49.37
Nonvested shares, number, ending balance		297,000,000	355,000,000	410,000,000
Nonvested shares, weighted average price, ending balance		$ 60.35	$ 32.35	$ 43.99
Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options		$ 7.5
Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		1 year 5 months
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Nonvested shares, number, beginning balance	1,516,000	1,593,000	1,429,000	1,326,000
Nonvested shares, weighted average price, beginning balance	$ 42.97	$ 38.86	$ 44.12	$ 53.18
Nonvested shares, granted		737,000	804,000	724,000
Nonvested shares, weighted average price, granted		$ 51.70	$ 40.10	$ 30.07
Nonvested shares, vested		(556,000)	(521,000)	(551,000)
Nonvested shares, weighted average price, vested		$ 42.25	$ 56.53	$ 47.55
Nonvested shares, forfeited		(258,000)	(119,000)	(70,000)
Nonvested shares, weighted average price, forfeited		$ 44.51	$ 38.97	$ 43.05
Nonvested shares, number, ending balance		1,516,000	1,593,000	1,429,000
Nonvested shares, weighted average price, ending balance		$ 42.97	$ 38.86	$ 44.12
Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options		$ 29.3
Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		2 years
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%
Performance share plan expense		$ 18.0	$ 15.9	$ 7.1
Performance share plan, shares issued in period		249,000	126,000	0
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Nonvested shares, number, beginning balance	297,000	355,000	320,000	0
Nonvested shares, weighted average price, beginning balance	$ 47.03	$ 39.42	$ 38.62	$ 0.00
Nonvested shares, granted		287,000	198,000	485,000
Nonvested shares, weighted average price, granted		$ 50.82	$ 40.08	$ 38.62
Nonvested shares, vested		(166,000)	(156,000)	(126,000)
Nonvested shares, weighted average price, vested		$ 38.62	$ 38.62	$ 38.62
Nonvested shares, forfeited		(179,000)	(7,000)	(39,000)
Nonvested shares, weighted average price, forfeited		$ 45.82	$ 39.20	$ 38.62
Nonvested shares, number, ending balance		297,000	355,000	320,000
Nonvested shares, weighted average price, ending balance		$ 47.03	$ 39.42	$ 38.62
Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options		$ 8.6
Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		1 year 5 months
Scenario, Forecast [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Plan Modification, Incremental Compensation Cost	$ 4.0