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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, 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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