Fair value measurements - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair value
Less than 12 months	$ 287,982	$ 67,217
12 months or greater	1,205	79,618
Total	289,187	146,835
Unrealized losses
Less than 12 months	(114)	(46)
12 months or greater	0	(45)
Total	(114)	(91)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	211,384	13,234
12 months or greater	0	79,618
Total	211,384	92,852
Unrealized losses
Less than 12 months	(74)	(3)
12 months or greater	0	(45)
Total	(74)	(48)
Corporate bonds
Fair value
Less than 12 months	76,598	53,983
12 months or greater	1,205	0
Total	77,803	53,983
Unrealized losses
Less than 12 months	(40)	(43)
12 months or greater	0	0
Total	$ (40)	$ (43)