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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Abstract]
United States federal tax at statutory rate	$ (7,049)	$ (13,410)	$ (91)
State taxes (net of federal benefit)	(897)	(1,608)	609
Deferred income tax adjustments	661	0	(855)
Deferred state blended rate adjustments	(493)	3,034	(344)
Research credit, net	(3,296)	(2,228)	(226)
Transaction cost deduction	0	(379)	0
Transaction cost deduction - prior year adjustment	0	(564)	0
Orphan drug credit, net	(106)	(139)	(843)
Other permanent items	(25)	(382)	3
Equity-based compensation	1,102	756	241
Fair value adjustment of preferred stock warrant liability	0	0	219
Change in valuation allowance	10,103	14,920	1,287
Income tax expense/(benefit)	$ 0	$ 0	$ 0

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deferred tax adjustments.

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