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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Income Taxes Disclosure [Line Items] |  |  |
Statutory income tax rate |  | 21.00% | 21.00%
Valuation allowance, increase (decrease) amount | $ 114,600,000 | $ (35,700,000) | $ (169,300,000)
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 | 10,511,000 | 17,295,000 | 8,038,000
Federal orphan drug credits | $ 17,700,000 |  |
Unrecognized tax benefits, period may increase or decrease due to tax examination | Next twelve months |  |
Unrecognized tax benefits, interest recognized | $ 0 | 0 | 0
Unrecognized tax benefits, penalties recognized | 0 | $ 0 | $ 0
Federal |  |  |
Income Taxes Disclosure [Line Items] |  |  |
Net operating loss carryforward for income tax | 1,721,700,000 |  |
Income tax research credits | 94,800,000 |  |
State |  |  |
Income Taxes Disclosure [Line Items] |  |  |
Net operating loss carryforward for income tax | 1,221,300,000 |  |
Income tax research credits | $ 49,400,000 |  |

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- Definition Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28680-109314 + Details
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