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Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
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 | 75,377,000 | 57,949,000 |
State net operating loss carryforward | 6,583,000 | 3,907,000 |
Research and development credit, net | 12,829,000 | 10,278,000 |
Orphan drug credit, net | 19,855,000 | 19,284,000 |
Deferred rent | 2,497,000 | 2,947,000 |
Deferred revenue | 5,098,000 | 6,632,000 |
Depreciation | 2,926,000 | 1,597,000 |
Other | 5,085,000 | 2,532,000 |
Gross deferred income tax assets | 130,250,000 | 105,126,000 |
Valuation allowance | (128,844,000) | (104,399,000) |
Net deferred income tax assets | 1,406,000 | 727,000 |
Deferred income tax liabilities: |  |  |
Prepaid expenditures | (1,406,000) | (727,000) |
Gross deferred income tax liabilities | (1,406,000) | (727,000) |
Net deferred income tax asset/(liability) | 0 | $ 0 |
Operating Loss Carryforwards [Line Items] |  |  |
Remaining portion of net operating losses | 201,900,000 |  |
Net operating losses, recognized as a benefit through additional-paid-in-capital | 18,600,000 |  |
U.S. Federal and State |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforwards | 215,400,000 |  |
U.S. Federal Government |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
US Federal tax credits carry forward | 32,500,000 |  |
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 |  |

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