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Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Income Tax Disclosure [Abstract] |  |  |
Provision for federal or state income taxes | $ 0 | $ 0 | $ 0
Deferred income tax assets: |  |  |
Federal U.S. net operating loss carryforward | 136,086,000 | 116,436,000 |
State net operating loss carryforward | 37,465,000 | 31,110,000 |
Research and development credit, net | 50,271,000 | 35,580,000 |
Orphan drug credit, net | 23,409,000 | 22,881,000 |
Operating lease liabilities | 8,048,000 | 8,413,000 |
Deferred revenue | 3,132,000 | 367,000 |
Other | 14,285,000 | 9,123,000 |
Gross deferred income tax assets | 272,696,000 | 223,910,000 |
Valuation allowance | (263,403,000) | (214,893,000) |
Net deferred income tax assets | 9,293,000 | 9,017,000 |
Deferred income tax liabilities: |  |  |
Depreciation | (2,123,000) | (438,000) |
Operating lease ROU assets | (5,316,000) | (5,547,000) |
Prepaid expenditures | (1,854,000) | (3,032,000) |
Gross deferred income tax liabilities | (9,293,000) | (9,017,000) |
Net deferred income tax asset/(liability) | 0 | $ 0 |
Operating Loss Carryforwards [Line Items] |  |  |
Remaining portion of net operating losses | 634,500,000 |  |
U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | 648,000,000.0 |  |
U.S. Federal Government |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
US Federal tax credits carry forward | 69,600,000 |  |
Net operating loss for limited use | 13,500,000 |  |
U.S. Federal Government | Minimum |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating losses limited for use utilized on an annual basis | 200,000 |  |
U.S. Federal Government | Maximum |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating losses limited for use utilized on an annual basis | 1,400,000 |  |
Tax Year 2025 To 2037 | U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | 237,700,000 |  |
Indefinite | U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | $ 410,300,000 |  |

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