Fair Value Measurements - Fair value hierarchy for its financial assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Level 1 [Member]
Assets
Total assets	$ 225,373	$ 42,960
Liabilities
Convertible notes payable	0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	225,373	42,960
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total assets	0	0
Level 1 [Member] | Treasury bills
Assets
Total assets	0	0
Level 1 [Member] | Corporate debt securities [Member]
Assets
Total assets	0	0
Level 2 [Member]
Assets
Total assets	152,141	20,000
Liabilities
Convertible notes payable	309,166	125,811
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	24,250	20,000
Level 2 [Member] | Treasury bills
Assets
Total assets	99,972	0
Level 2 [Member] | Corporate debt securities [Member]
Assets
Total assets	27,919	0
Level 3 [Member]
Assets
Total assets	0	0
Liabilities
Convertible notes payable	0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total assets	0	0
Level 3 [Member] | Treasury bills
Assets
Total assets	0	0
Level 3 [Member] | Corporate debt securities [Member]
Assets
Total assets	$ 0	$ 0