Income Taxes - Schedule of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Accrued expenses and other	$ 4,349	$ 1,674
Prepaid licensing arrangement	11,154	11,562
Interest expense	4,469	1,267
Stock compensation	5,468	3,493
Research and development credits	2,485	2,485
Net operating losses	51,644	32,548
Total deferred tax assets	79,569	53,029
Deferred tax liabilities:
Fixed assets, including leases	(630)	(409)
Total deferred tax liabilities:	(630)	(409)
Less: Valuation allowance	(78,939)	(52,620)
Total net deferred tax assets (liabilities)	$ 0	$ 0