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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States federal tax at statutory rate	$ (6,869)	$ (20,489)	$ (7,049)
State taxes (net of federal benefit)	(735)	(3,116)	(897)
Deferred income tax adjustments	607	173	661
Deferred state blended rate adjustments	(485)	(32)	(493)
Deferred federal rate change reduction in corporate rate	39,447	0	0
Research credit, net	(8,455)	(2,551)	(3,296)
Orphan drug credit, net	(1,853)	(571)	(106)
Other permanent items	276	145	(25)
Equity-based compensation	2,067	1,997	1,102
Change in valuation allowance	(24,000)	24,444	10,103
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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