Share-Based Compensation - Stock Options Activity (Details) - Stock Option - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Shares
Beginning balance (in shares)	1,924,683
Granted (in shares)	300,495
Exercised (in shares)	(104,754)
Forfeited/Expired (in shares)	(16,746)
Ending balance (in shares)	2,103,678	1,924,683
Vested and expected to vest (in shares)	1,967,982
Exercisable (in shares)	1,149,993
Weighted Average Price
Beginning Balance, Weighted Average Price (in dollars per share)	$ 71.07
Granted, Weighted Average Price (in dollars per share)	138.12
Exercised, Weighted Average Price (in dollars per share)	62.91
Forfeited, Weighted Average Price (in dollars per share)	90.72
Ending Balance, Weighted Average Price (in dollars per share)	$ 80.90	$ 71.07
Weighted-Average Remaining Contractual Term
Stock options outstanding, Weighted-Average Remaining Contractual Term	7 years 1 month 13 days	6 years 10 months 13 days
Aggregate Intrinsic Value
Stock options outstanding, Aggregate intrinsic value	$ 129,612,247	$ 117,209,218
Vested and expected to vest , Weighted Average Price (in dollars per share)	$ 79.10
Exercisable, Weighted Average Price (in dollars per share)	$ 63.95
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 11 months 27 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 9 months 7 days
Aggregate Intrinsic Value, Vested and expected to vest	$ 124,784,752
Aggregate Intrinsic Value, Exercisable	$ 90,343,139