Fair Value Measurements - Fair value hierarchy for its financial assets and liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Level 1 [Member]
Assets
Total assets	$ 40,354	$ 39,168
Liabilities
Convertible notes payable	0	0
Level 1 [Member] | Money market funds [Member]
Assets
Total assets	40,354	39,168
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	25,990	46,216
Liabilities
Convertible notes payable	120,530	197,935
Level 2 [Member] | Money market funds [Member]
Assets
Total assets	0	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total assets	15,000	19,997
Level 2 [Member] | Corporate debt securities [Member]
Assets
Total assets	10,990	26,219
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