Income Taxes - Benefit for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the expected statutory tax benefit to the actual income taxes
U.S. federal corporate tax rate (as a percent)	21.00%	34.00%	34.00%
Loss before income tax expense	$ (104,133)	$ (168,843)	$ (96,012)
Expected tax benefit at 21%, 34%, 34% and 34%, respectively	(21,868)	(35,457)	(32,644)
Permanent differences	320	(103)	25
Incentive stock options	569	1,144	1,528
State tax benefit net of federal benefit	(6,726)	(10,622)	(3,537)
Change in valuation allowance, net	27,812	53,706	(63,238)
Federal research credit	(1,652)	(2,466)	(2,204)
Federal orphan drug credit	(4,426)	(6,934)	(7,118)
Expired loss and credit carryforwards	500
Change in U.S. tax law			97,479
Debt inducement			8,044
Lease incentive		109
Stock option expirations	5,471	623	1,665
Benefit for income taxes	$ 0	$ 0	$ 0