Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
United States federal tax at statutory rate	$ (20,489)	$ (7,049)	$ (13,410)
State taxes (net of federal benefit)	(3,116)	(897)	(1,608)
Deferred income tax adjustments	173	661	0
Deferred state blended rate adjustments	(32)	(493)	3,034
Research credit, net	(2,551)	(3,296)	(2,228)
Transaction cost deduction	0	0	(379)
Transaction cost deduction - prior year adjustment	0	0	(564)
Orphan drug credit, net	(571)	(106)	(139)
Other permanent items	145	(25)	(382)
Equity-based compensation	1,997	1,102	756
Change in valuation allowance	24,444	10,103	14,920
Income tax expense/(benefit)	$ 0	$ 0	$ 0