Share-Based Compensation - Stock Options Activity (Details) - Stock Option - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding at January 1, 2017 (in shares)	1,987,946
Granted (in shares)	441,990
Exercised (in shares)	(448,189)
Forfeited (in shares)	(57,064)
Outstanding at December 31, 2017 (in shares)	1,924,683	1,987,946
Vested and expected to vest at December 31, 2017 (in shares)	1,823,279
Exercisable at December 31, 2017 (in shares)	898,003
Weighted Average Price
Outstanding at January 1, 2016 (in dollars per share)	$ 61.83
Granted (in dollars per share)	93.48
Exercised (in dollars per share)	51.17
Forfeited (in dollars per share)	79.14
Outstanding at December 31, 2017 (in dollars per share)	71.07	$ 61.83
Vested and expected to vest at December 31, 2017 (in dollars per share)	70.26
Exercisable at December 31, 2017 (in dollars per share)	$ 57.21
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term	6 years 10 months 13 days	6 years 10 months 2 days
Vested and expected to vest at December 31, 2017	6 years 9 months 7 days
Exercisable at December 31, 2017	5 years 3 months 7 days
Aggregate Intrinsic Value
Outstanding at January 1, 2017	$ 56,144,876
Outstanding at December 31, 2017	117,209,218	$ 56,144,876
Vested and expected to vest at December 31, 2017	112,521,086
Exercisable at December 31, 2017	$ 67,130,223