Fair Value Measurements - Fair value hierarchy for its financial assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Level 1 [Member]
Assets
Total assets	$ 42,960	$ 39,168
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	42,960	39,168
Level 2 [Member]
Assets
Total assets	20,000	46,216
Liabilities
Convertible notes payable	125,811	197,935
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	$ 20,000	19,997
Level 2 [Member] | Corporate debt securities [Member]
Assets
Total assets		$ 26,219