Stock-Based Compensation - Summary Of Changes In SARs Outstanding (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost not yet recognized, period for recognition	2 years 10 months 24 days
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Award expiration period from date of grant	7 years
Total compensation cost not yet recognized, period for recognition	4 years
SARs [Roll Forward]
Outstanding at December 31, 2017 (in shares)	1.2
Granted (in shares)	0.3
Outstanding at March 31, 2018 (in shares)	1.5	1.2
Vested and exercisable at March 31, 2018 (in shares)	0.8
Aggregate Intrinsic value of outstanding shares	$ 47.6
Aggregate intrinsic value of shares vested and exercisable	$ 35.1
Per Share Weighted Average Exercise Price [Roll Forward]
Outstanding at December 31, 2017 (in dollars per share)	$ 76.73
Granted (in dollars per share)	114.26
Outstanding at March 31, 2018 (in dollars per share)	85.06	$ 76.73
Vested and exercisable at March 31, 2018 (in dollars per share)	70.96
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at December 31, 2017 (in dollars per share)	17.35
Granted (in dollars per share)	25.63
Outstanding at March 31, 2018 (in dollars per share)	19.15	$ 17.35
Vested and exercisable at March 31, 2018 (in dollars per share)	$ 16.39
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Remaining Contractual Term [Abstract]
Granted	6 years 10 months 10 days
Outstanding weighted average remaining contractual term	4 years 8 months 12 days	4 years 3 months 12 days
Vested and exercisable at March 31, 2018	3 years 6 months 22 days
Unvested | Stock appreciation rights
SARs [Roll Forward]
Outstanding at March 31, 2018 (in shares)	0.7