Stock-based Compensation - Restricted Stock and Stock Options (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in unvested stock options
Compensation expense	$ 23,364,000	$ 16,881,000	$ 3,513,000
Restricted Stock
Changes in unvested restricted stock
Unvested restricted common stock, beginning of period (in shares)	822,638
Issued (in shares)	480,000
Vested (in shares)	(784,119)
Forfeited (in shares)	(5,294)
Unvested restricted common stock, end of period(in shares)	513,225	822,638
Weighted Average Grant Date Fair Value
Balance, beginning of period	$ 0.02
Issued (in dollars per share)	28.05
Vested (in dollars per share)	4.95
Forfeited (in dollars per share)	0.03
Balance, ending of period	$ 18.70	$ 0.02
Restricted Stock | Employees
Changes in unvested stock options
Compensation expense	$ 500,000	$ 0	0
Unrecognized stock-based compensation expense	0
Restricted Stock | Non-employees
Changes in unvested stock options
Compensation expense	4,100,000	$ 8,300,000	2,300,000
Unrecognized stock-based compensation expense	$ 10,300,000
Period for recognition	4 years 8 months 12 days
Employee and Consultant Options
Changes in unvested stock options
Outstanding, beginning of period (in shares)	3,411,783
Granted (in shares)	1,392,689
Exercised (in shares)	(289,583)			(75,304)
Cancelled (in shares)	(142,753)
Outstanding, end of period (in shares)	4,372,136	3,411,783
Vested and expected to vest (in shares)	4,372,126
Exercisable (in shares)	1,540,023
Outstanding, beginning of period (in dollars per share)	$ 13.71
Exercised (in dollars per share)				$ 0.03
Outstanding, end of period (in dollars per share)	17.28	$ 13.71
Vested and expected to vest outstanding, weighted average exercise price (in dollars per share)	17.28
Exercisable (in dollar per share)	$ 14.95
Remaining contractual life	8 years 6 months	8 years 9 months 18 days
Vested and expected to vest, remaining contractual life	8 years 6 months
Exercisable, remaining contractual life	8 years 3 months 18 days
Aggregate intrinsic value	$ 60,591,000	$ 16,190,000
Vesting and expected to vest, aggregate intrinsic value	60,591,000
Exercisable, aggregated intrinsic value	$ 25,099,000
Unvested stock options (in shares)	4,572	21,955
Intrinsic value of options exercised	$ 5,000,000	$ 900,000	$ 100,000
Weighted average fair value of options granted (per share)	$ 16.07	$ 14.10	$ 5.91
Employee and Consultant Options | Employees and directors
Changes in unvested stock options
Compensation expense	$ 12,300,000	$ 6,000,000	$ 700,000
Employee and Consultant Options | Employees
Changes in unvested stock options
Unrecognized stock-based compensation expense	$ 33,400,000
Period for recognition	2 years 6 months