Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 70,317	$ 49,699
Tax credit carryforwards	11,966	10,160
Capitalized start-up costs	695	870
Capitalized research and development expenses, net	14,521	18,418
Operating lease liabilities	2,505
Derivative liability	3,028
Accrued expenses and other temporary differences	6,987	5,196
Total gross deferred tax assets	110,019	84,343
Valuation allowance	(105,062)	$ (84,343)	$ (67,726)	$ (70,236)
Net deferred tax assets	4,957
Operating lease right of use assets	(1,662)
2026 Convertible Notes	(3,295)
Total deferred tax liabilities	$ (4,957)