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Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 | 116,436,000 | 87,284,000 |
State net operating loss carryforward | 31,110,000 | 22,809,000 |
Research and development credit, net | 35,580,000 | 29,750,000 |
Orphan drug credit, net | 22,881,000 | 22,580,000 |
Operating lease liabilities | 8,413,000 | 0 |
Deferred revenue | 367,000 | 3,736,000 |
Other | 9,123,000 | 9,972,000 |
Gross deferred income tax assets | 223,910,000 | 176,131,000 |
Valuation allowance | (214,893,000) | (172,457,000) |
Net deferred income tax assets | 9,017,000 | 3,674,000 |
Deferred income tax liabilities: |  |  |
Depreciation | (438,000) | (1,911,000) |
Operating lease ROU assets | (5,547,000) | 0 |
Prepaid expenditures | (3,032,000) | (1,763,000) |
Gross deferred income tax liabilities | (9,017,000) | (3,674,000) |
Net deferred income tax asset/(liability) | 0 | $ 0 |
Operating Loss Carryforwards [Line Items] |  |  |
Remaining portion of net operating losses | 541,000,000.0 |  |
U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | 554,500,000 |  |
U.S. Federal Government |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
US Federal tax credits carry forward | 58,200,000 |  |
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 | 237,800,000 |  |
Indefinite | U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | $ 316,700,000 |  |

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