Share-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Number of shares
Granted		0
Additional disclosure
Weighted average grant-date fair value per share	$ 4.87
Total fair value of options vested	$ 2,631	$ 0
Plans
Number of shares
Outstanding at the beginning	3,864,425
Granted	1,362,987
Exercised	(182,615)
Forfeited	(11,760)
Outstanding at the end	5,033,037		3,864,425
Options exercisable as of September 30, 2017	2,454,181
Options unvested as of September 30, 2017	2,578,856
Weighted Average Exercise Price
Outstanding at the beginning (in dollars per share)	$ 3.61
Granted (in dollars per share)	15.15
Exercised (in dollars per share)	1.39
Forfeited (in dollars per share)	4.44
Outstanding at the end (in dollars per share)	6.81		$ 3.61
Options exercisable as of September 30, 2017 (in dollars per share)	3.73
Options unvested as of September 30, 2017 (in dollars per share)	$ 9.74
Additional disclosure
Outstanding average remaining contractual term (in years)	9 years 2 months 9 days		9 years 2 months 16 days
Granted weighted average remaining contractual term (in years)	9 years 6 months 18 days
Options exercisable as of September 30, 2017 (in years)	7 years 10 months 2 days
Options unvested as of September 30, 2017 ( in years)	8 years 8 months 9 days
Outstanding aggregate intrinsic value	$ 153,889		$ 15,991
Options exercisable as of September 30, 2017 (in dollars)	82,579
Options unvested as of September 30, 2017 (in dollars)	$ 71,310