Summary of significant accounting policies - Cash, Cash Equivalents, and Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Cash and cash equivalents	$ 104,440	$ 86,042	$ 46,859
Marketable securities, amortized cost	71,568	196,687
Total money market funds and marketable securities	175,560	274,990
Fair Value
Marketable securities, fair value	76,698	197,411
Total money market funds and marketable securities	180,690	275,714
Unrealized Gains and Losses
Marketable securities, unrealized gains	5,136	752
Total money market funds and marketable securities	5,136	752
Marketable securities, unrealized losses	6	28
Total money market funds and marketable securities	6	$ 28
Other than temporary losses	$ 0
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 103,992	$ 78,303
Fair Value
Cash and cash equivalents	103,992	78,303
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	70,348	195,467
Fair Value
Marketable securities, fair value	70,342	195,491
Unrealized Gains and Losses
Marketable securities, unrealized gains		52
Marketable securities, unrealized losses	6	28
Equity securities
Amortized Cost
Marketable securities, amortized cost	1,220	1,220
Fair Value
Marketable securities, fair value	6,356	1,920
Unrealized Gains and Losses
Marketable securities, unrealized gains	$ 5,136	$ 700