Share-Based Compensation - Restricted Stock Options Activity and Status (Details) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) item $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item $ / shares shares	Sep. 30, 2018 USD ($)
Aggregate Intrinsic Value
Share-based compensation expense (benefit) before tax | $	$ 23.9	$ 51.4	$ 15.9	$ (29.2)
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		1
Number of Restricted Stock Units
Unvested at beginning of period (in shares) | shares			186,255
Granted (in shares) | shares			222,614
Vested (in shares) | shares			(69,761)
Forfeited/cancelled (in shares) | shares			(18,445)
Unvested at the end of period (in shares) | shares	320,663		320,663
Weighted Average Grant Price
Unvested at beginning of period (in dollars per share) | $ / shares			$ 112.48
Granted (in dollars per share) | $ / shares			113.03
Vested (in dollars per share) | $ / shares			111.81
Forfeited/cancelled (in dollars per share) | $ / shares			118.24
Unvested at the end of period (in dollars per share) | $ / shares	$ 112.68		$ 112.68
Weighted Average Remaining Contractual Term (Years)
Outstanding at the end of period (Years)			9 years 2 months 12 days
Aggregate Intrinsic Value
Unvested at the end of period (in dollars) | $	$ 25.6		$ 25.6
Share-based compensation expense (benefit) before tax | $	$ 3.8	$ 2.4	$ 9.8	$ 5.3
Weighted average remaining vesting period (in years)			2 years 1 month 6 days