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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
United States federal tax at statutory rate	$ (36,005)	$ (6,869)	$ (20,489)
State taxes (net of federal benefit)	(11,133)	(735)	(3,116)
Deferred income tax adjustments	(4,435)	607	173
Deferred state blended rate adjustments	0	(485)	(32)
Deferred federal rate change reduction in corporate rate	0	39,447	0
Research credit, net	(8,466)	(8,455)	(2,551)
Orphan drug credit, net	(872)	(1,853)	(571)
Other permanent items	148	276	145
Equity-based compensation	758	2,067	1,997
Change in valuation allowance	60,005	(24,000)	24,444
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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