Subsequent events (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Nov. 27, 2017	Oct. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Oct. 12, 2017
Subsequent Event [Line Items]
Stock-based compensation expense			$ 771	$ 34	$ 1,800	$ 539
Subsequent Event
Subsequent Event [Line Items]
Accrued expenses for contractors, non-refundable portion	$ 15,500
Subsequent Event | IPO
Subsequent Event [Line Items]
Price per share (usd per share)		$ 16.00
Gross proceeds from IPO		$ 138,000
Net proceeds from IPO		125,540
Fees and commissions		9,660
Stock issuance costs		$ 2,800
Subsequent Event | Amended and Restated 2010 Stock Incentive Plan
Subsequent Event [Line Items]
Granted (in shares)		1,599,881
Granted, Weighted average exercise price (in dollars per share)		$ 16.03
Fair value of common stock (in dollars per share)		$ 11.18
Subsequent Event | Amended and Restated 2010 Stock Incentive Plan | Performance shares
Subsequent Event [Line Items]
Stock-based compensation expense		$ 1,642
Subsequent Event | Amended and Restated 2010 Stock Incentive Plan | Employee Stock
Subsequent Event [Line Items]
Shares to be issued under purchase plan (in shares)							144,395
Minimum | Subsequent Event
Subsequent Event [Line Items]
Early termination fees	100
Maximum | Subsequent Event
Subsequent Event [Line Items]
Early termination fees	$ 700