Stock options - Activity under stock option plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Number of Shares
Outstanding at beginning of period (in shares)	3,205,485
Granted (in shares)	984,274
Exercised (in shares)	(104,945)
Cancelled (in shares)	(178,969)
Outstanding at end of period (in shares)	3,905,845		3,205,485
Options exercisable, at end of period (in shares)	1,693,746
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 3.44
Granted (in dollars per share)	14.37
Exercised (in dollars per share)	2.43
Cancelled (in dollars per share)	7.71
Outstanding at end of period (in dollars per share)	6.02		$ 3.44
Options exercisable, at end of period (in dollars per share)	$ 2.25
Additional Disclosures
Remaining Contractual Life, Options outstanding	7 years 8 months 12 days		7 years 9 months 18 days
Remaining Contractual Life, Options exercisable	6 years 2 months 12 days
Aggregate Intrinsic Value, Options outstanding (in dollars)	$ 38,254		$ 41,709
Aggregate Intrinsic Value, Options exercisable (in dollars)	$ 22,908
Weighted-average grant date fair value of options granted (in dollars per share)	$ 9.47	$ 4.22
Cash received from the exercise of stock options (in dollars)	$ 255	$ 104