Stock-based compensation - 2018 Stock Plan - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended			9 Months Ended		10 Months Ended
	Mar. 05, 2020	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Mar. 31, 2020	Mar. 06, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from exercises of stock options, net of shares withheld for taxes					$ 619	$ 78
2018 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated vesting. number of shares (in shares)	2,600
Accelerated cost	$ 2,500
Stock options vested (in shares)	1,700
Shares options nonvested (in shares)	100
Exercise price range, lower limit (in dollars per share)					$ 3.51
Exercise price range, upper limit (in dollars per share)					$ 3.54
Options forfeited (in shares)					70
Exercise of stock options (in shares)		687	257	37	1,010		28
Proceeds from exercises of stock options, net of shares withheld for taxes					$ 2,100
Number of shares exercisable (in shares)		3,500			3,500
Average exercise price of shares exercisable (in dollars per share)		$ 3.54			$ 3.54
Options outstanding (in shares)		3,603	4,291	4,552	3,603		4,600	1,887	4,678	2,734
Options outstanding, intrinsic value		$ 92,500			$ 92,500
Remaining contractual term					7 years 2 months 23 days
2018 Stock Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares withheld (in shares)					250
Unrecognized compensation expense, options		$ 200			$ 200
Period for recognition					1 year 11 months 4 days