Fair Value Measurements - Fair value hierarchy for its financial assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Level 1 [Member]
Assets
Total assets	$ 47,654	$ 42,960
Liabilities
Convertible notes payable	0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	47,654	42,960
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total assets	0	0
Level 1 [Member] | Corporate debt securities [Member]
Assets
Total assets	0	0
Level 2 [Member]
Assets
Total assets	150,353	20,000
Liabilities
Convertible notes payable	253,331	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] | Corporate debt securities [Member]
Assets
Total assets	126,103	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] | Corporate debt securities [Member]
Assets
Total assets	$ 0	$ 0