Share-Based Compensation - Restricted Stock Options Activity and Status (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) item $ / shares shares	Mar. 31, 2018 USD ($)
Aggregate Intrinsic Value
Share-based compensation expense (benefit) before tax | $	$ 29.2	$ (101.1)
Restricted Stock Units
Share-Based Compensation
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Entitled To Receive For Each Unit Upon Vesting | item	1
Aggregate Intrinsic Value
Share-based compensation expense (benefit) before tax | $	$ 2.2	0.9
Restricted Stock Units | 2015 Plan
Number of Restricted Stock Units
Unvested at beginning of period (in shares) | shares	186,255
Granted (in shares) | shares	196,489
Vested (in shares) | shares	(46,027)
Forfeited/cancelled (in shares) | shares	(4,373)
Unvested at the end of period (in shares) | shares	332,344
Weighted Average Grant Price
Unvested at beginning of period (in dollars per share) | $ / shares	$ 112.48
Granted (in dollars per share) | $ / shares	117.75
Vested (in dollars per share) | $ / shares	111.04
Forfeited/cancelled (in dollars per share) | $ / shares	119.39
Unvested at the end of period (in dollars per share) | $ / shares	$ 115.71
Weighted Average Remaining Contractual Term (Years)
Outstanding at the end of period (Years)	9 years 7 months 6 days
Aggregate Intrinsic Value
Unvested at the end of period (in dollars) | $	$ 39.0
Share-based compensation expense (benefit) before tax | $	$ 2.2	$ 0.9
Weighted average remaining vesting period (in years)	2 years 7 months 6 days