Stock incentive plan - Restricted stock units (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	May 31, 2020	May 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Weighted-average grant date fair value per share
Non-cash restricted stock unit award expense recognized			$ 2,503	$ 2,325	$ 4,834	$ 4,325
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		2 years
Shares
Unvested at beginning of period (in shares)					1,032,000
Issued (in shares)	205,200	1,183,400			205,000
Vested					(21,000)
Forfeited and canceled (in shares)					(85,000)
Unvested at end of period (in shares)			1,130,800		1,130,800
Weighted-average grant date fair value per share
Unvested at beginning of period (in dollars per share)					$ 3.63
Issued (in dollars per share)					3.61
Vested (in dollars per share)					3.63
Forfeited and canceled (in dollars per share)					3.63
Unvested at end of period (in dollars per share)			$ 3.63		$ 3.63
Non-cash restricted stock unit award expense recognized			$ 500	$ 300	$ 900	$ 300
Unrecognized compensation related to unvested restricted stock units			$ 2,100		$ 2,100
Expenses recognized over a weighted-average period (in years)					9 months 18 days