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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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Present value of lessee's discounted obligation for lease payments from operating lease.

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