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Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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 | 87,284,000 | 50,346,000 |
State net operating loss carryforward | 22,809,000 | 7,551,000 |
Research and development credit, net | 29,750,000 | 21,284,000 |
Orphan drug credit, net | 22,580,000 | 21,708,000 |
Deferred rent | 3,118,000 | 3,385,000 |
Deferred revenue | 3,736,000 | 2,982,000 |
Depreciation | 0 | 155,000 |
Other | 6,854,000 | 5,847,000 |
Gross deferred income tax assets | 176,131,000 | 113,258,000 |
Valuation allowance | (172,457,000) | (112,453,000) |
Net deferred income tax assets | 3,674,000 | 805,000 |
Deferred income tax liabilities: |  |  |
Depreciation | (1,911,000) | 0 |
Prepaid expenditures | (1,763,000) | (805,000) |
Gross deferred income tax liabilities | (3,674,000) | (805,000) |
Net deferred income tax asset/(liability) | 0 | $ 0 |
Operating Loss Carryforwards [Line Items] |  |  |
Remaining portion of net operating losses | 402,100,000 |  |
Previously unrecognized excess tax benefits | 18,600,000 |  |
Previously unrecognized excess tax benefits, tax effected portion | 6,500,000 |  |
U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | 415,600,000 |  |
U.S. Federal Government |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
US Federal tax credits carry forward | 52,300,000 |  |
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 | $ 177,800,000 |  |

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