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Summary of Significant Accounting Policies - Additional Information (Detail) |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) Segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016
Accounting Policy [Line Items] |  |  |  |
Number of operating segment | Segment |  | 1 |  |
Highly liquid investment maturity period |  | three months or less |  |
Restricted time deposits |  | $ 4,145,000 | $ 5,181,000 |
Cash and cash equivalents |  | $ 89,258,000 | $ 673,658,000 |
Short term investments Maturity |  | 12 months |  |
Long term Investments Maturity |  | 12 months |  |
Impairment of long-lived assets |  | $ 0 |  |
U.S. federal statutory income tax rate |  | 21.00% | 34.00% | 34.00%
ASU 2018-02 [Member] | Accumulated Deficit [Member] | Subsequent Event [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Cumulative effect on accumulated deficit | $ 600,000 |  |  |
Minimum [Member] | ASU 2016-02 [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Right-of-use assets |  | $ 47,000,000 |  |
Lease liabilities |  | $ 57,000,000 |  |
Minimum [Member] | Computer Equipment [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 3 years |  |
Minimum [Member] | Laboratory Equipment [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 3 years |  |
Minimum [Member] | Furniture and Fixtures [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 3 years |  |
Maximum [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
U.S. federal statutory income tax rate |  |  | 35.00% |
Maximum [Member] | ASU 2016-02 [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Right-of-use assets |  | $ 62,000,000 |  |
Lease liabilities |  | $ 67,000,000 |  |
Maximum [Member] | Computer Equipment [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 5 years |  |
Maximum [Member] | Laboratory Equipment [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 5 years |  |
Maximum [Member] | Furniture and Fixtures [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Property and equipment estimated useful life |  | 5 years |  |
Foreign subsidiaries [Member] |  |  |  |
Accounting Policy [Line Items] |  |  |  |
Cash and cash equivalents |  | $ 21,900,000 |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3044-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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