Share-Based Compensation - Stock Options Activity (Details) - Stock Option - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Shares
Beginning balance (in shares)	1,714,003
Granted (in shares)	334,430
Exercised (in shares)	(298,167)
Forfeited/Expired (in shares)	(61,098)
Ending balance (in shares)	1,689,168	1,714,003
Vested and expected to vest (in shares)	1,587,297
Exercisable (in shares)	860,924
Weighted Average Price
Beginning Balance, Weighted Average Price (in dollars per share)	$ 85.08
Granted, Weighted Average Price (in dollars per share)	144.89
Exercised, Weighted Average Price (in dollars per share)	69.54
Forfeited, Weighted Average Price (in dollars per share)	112.34
Ending Balance, Weighted Average Price (in dollars per share)	$ 98.68	$ 85.08
Weighted-Average Remaining Contractual Term
Stock options outstanding, Weighted-Average Remaining Contractual Term	7 years 21 days	6 years 8 months 12 days
Aggregate Intrinsic Value
Stock options outstanding, Aggregate intrinsic value	$ 124,043,143	$ 74,191,783
Vested and expected to vest , Weighted Average Price (in dollars per share)	$ 95.88
Exercisable, Weighted Average Price (in dollars per share)	$ 75.87
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 10 months 17 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 7 months 20 days
Aggregate Intrinsic Value, Vested and expected to vest	$ 121,043,268
Aggregate Intrinsic Value, Exercisable	$ 82,885,109