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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Tax Disclosure [Abstract] |  |  |
United States federal tax at statutory rate | $ (31,880) | $ (36,005) | $ (6,869)
State taxes (net of federal benefit) | (9,524) | (11,133) | (735)
Deferred income tax adjustments | 2,004 | (4,435) | 607
Deferred state blended rate adjustments | 0 | 0 | (485)
Deferred federal rate change reduction in corporate rate | 0 | 0 | 39,447
Research credit, net | (5,830) | (8,466) | (8,455)
Orphan drug credit, net | (301) | (872) | (1,853)
Other permanent items | 1,206 | 148 | 276
Equity-based compensation | 1,889 | 758 | 2,067
Change in valuation allowance | 42,436 | 60,005 | (24,000)
Income tax expense/(benefit) | $ 0 | $ 0 | $ 0

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