Stock-based compensation - Employee Stock Purchase Plan (Details) $ in Thousands		12 Months Ended
Dec. 18, 2018 item shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares
Stock-based compensation
Shares reserved for issuance \| shares		41,457,000	39,129,000
Assumptions used to estimate fair value of each stock-based award on the grant date
Stock-based compensation expense		$ 7,711	$ 8,118	$ 8,539
Proceeds from the issuance of common stock, net		$ 6,723	$ 106,059
Employee Stock purchase plan
Stock-based compensation
Percent of common stock at market price to be purchased	85.00%
Number of vesting periods \| item	2
Vesting period	6 months
Shares reserved for issuance \| shares	2,000,000	1,190,000	1,300,000
Assumptions used to estimate fair value of each stock-based award on the grant date
Risk-free interest rate (as a percent)		0.07%	1.04%	2.26%
Volatility (as a percent)		68.00%	109.00%	88.00%
Expected term		6 months	6 months	6 months
Stock-based compensation expense		$ 100	$ 100	$ 400
Issuance of common stock under ESPP \| shares		110,060	358,193	341,701
Proceeds from the issuance of common stock, net		$ 200	$ 400	$ 400

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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