Share-Based Compensation - Stock Options Activity (Details) - Stock Option - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Shares
Beginning balance (in shares)	1,714,003
Granted (in shares)	332,625
Exercised (in shares)	(131,565)
Forfeited/Expired (in shares)	(44,168)
Ending balance (in shares)	1,870,895	1,714,003
Vested and expected to vest (in shares)	1,747,058
Exercisable (in shares)	1,023,778
Weighted Average Price
Beginning Balance, Weighted Average Price (in dollars per share)	$ 85.08
Granted, Weighted Average Price (in dollars per share)	144.83
Exercised, Weighted Average Price (in dollars per share)	67.42
Forfeited, Weighted Average Price (in dollars per share)	108.13
Ending Balance, Weighted Average Price (in dollars per share)	$ 96.40	$ 85.08
Weighted-Average Remaining Contractual Term
Stock options outstanding, Weighted-Average Remaining Contractual Term	7 years 1 month 13 days	6 years 8 months 12 days
Aggregate Intrinsic Value
Stock options outstanding, Aggregate intrinsic value	$ 103,532,580	$ 74,191,783
Vested and expected to vest , Weighted Average Price (in dollars per share)	$ 94.14
Exercisable, Weighted Average Price (in dollars per share)	$ 75.01
Weighted-Average Remaining Contractual Term, Vested and expected to vest	6 years 11 months 27 days
Weighted-Average Remaining Contractual Term, Exercisable	5 years 8 months 27 days
Aggregate Intrinsic Value, Vested and expected to vest	$ 100,638,073
Aggregate Intrinsic Value, Exercisable	$ 78,549,384