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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Taxes Disclosure [Line Items] |  |  |  |
Statutory income tax rate |  | 35.00% |  |
Valuation allowance, increase (decrease) amount |  | $ (169,300,000) | $ 52,500,000 | $ 17,300,000
Valuation allowance related to stock-based compensation included in income tax expense |  | 35,600,000 | 35,600,000 |
Additional income taxes for remaining undistributed foreign earnings |  | 0 |  |
Income tax research credits |  | 8,038,000 | $ 4,543,000 | $ 4,529,000
Federal orphan drug credits |  | $ 17,700,000 |  |
Federal orphan drug credits, expiration year |  | 2026 |  |
Unrecognized tax benefits, period may increase or decrease due to tax examination |  | Next twelve months |  |
Federal [Member] |  |  |  |
Income Taxes Disclosure [Line Items] |  |  |  |
Net operating loss carryforward for income tax |  | $ 1,610,300,000 |  |
Net operating loss carryforward, expiration year |  | 2018 |  |
Income tax research credits |  | $ 64,000,000 |  |
Income tax research credits, expiration year |  | 2019 |  |
State [Member] |  |  |  |
Income Taxes Disclosure [Line Items] |  |  |  |
Net operating loss carryforward for income tax |  | $ 474,000,000 |  |
Net operating loss carryforward, expiration year |  | 2019 |  |
Income tax research credits |  | $ 33,300,000 |  |
Scenario Forecast [Member] |  |  |  |
Income Taxes Disclosure [Line Items] |  |  |  |
Statutory income tax rate | 21.00% |  |  |

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