Stock-based Compensation - Service-based stock options (Details) - Service Based Stock Options - $ / shares	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Service-based stock options activity
Shares outstanding, beginning (in shares)	6,251,576
Granted (in shares)	358,192	334,993
Exercised (in shares)	(446,367)
Expired (in shares)	0
Forfeited (in shares)	(36,841)
Shares outstanding, ending (in shares)	6,126,560		6,251,576
Exercisable at September 30, 2017 (in shares)	3,114,397
Vested and expected to vest at September 30, 2017 (in shares)	6,126,560
Service-based stock options weighted average exercise price
Beginning balance, Weighted average exercise price (in dollars per share)	$ 9.34
Granted, Weighted average exercise price (in dollars per share)	19.98
Exercised, Weighted average exercise price (in dollars per share)	2.28
Expired, Weighted average exercise price (in dollars per share)	0.00
Forfeited, Weighted average exercise price (in dollars per share)	6.49
Ending balance, Weighted average exercise price (in dollars per share)	10.50		$ 9.34
Options exercisable, Weighted average exercise price per share (in dollars per share)	6.80
Vested and expected to vest, Weighted average exercise price per share (in dollars per share)	$ 10.50
Service-based stock options, additional disclosures
Options outstanding, Weighted average remaining contractual life	6 years 10 months 28 days		6 years 8 months 2 days
Options exercisable, Weighted average remaining contractual life	4 years 8 months 27 days
Vested and expected to vest, Weighted average remaining contractual life	6 years 10 months 28 days