Fair Value Measurements - Fair value hierarchy for its financial assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Level 1
Assets
Total cash equivalents and marketable securities	$ 96,116	$ 42,960
Liabilities
Convertible notes payable	0	0
Level 1 | Money market funds
Assets
Total cash equivalents and marketable securities	96,116	42,960
Level 1 | Government-backed securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Treasury securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Agency securities
Assets
Total cash equivalents and marketable securities	0	0
Level 2
Assets
Total cash equivalents and marketable securities	492,619	20,000
Liabilities
Convertible notes payable	407,238	125,811
Level 2 | Money market funds
Assets
Total cash equivalents and marketable securities	0	0
Level 2 | Government-backed securities
Assets
Total cash equivalents and marketable securities	44,250	20,000
Level 2 | Treasury securities
Assets
Total cash equivalents and marketable securities	54,088	0
Level 2 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	373,681	0
Level 2 | Agency securities
Assets
Total cash equivalents and marketable securities	20,600	0
Level 3
Assets
Total cash equivalents and marketable securities	0	0
Liabilities
Convertible notes payable	0	0
Level 3 | Money market funds
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Government-backed securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Treasury securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Agency securities
Assets
Total cash equivalents and marketable securities	$ 0	$ 0