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Short-term Investments - Additional Information (Detail) - US Treasury Securities and Certificates of Deposit - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Available-for-sale securities, amortized cost	$ 466,600,000
Available-for-sale securities, fair value	466,200,000
Unrecognized holding losses accumulated in other comprehensive loss, gross	400,000
Available-for-sale securities, other-than-temporary impairments related to these investments	$ 0
Held to maturity securities, amortized cost		$ 254,900,000
Held to maturity securities, carrying value		254,900,000
Held to maturity securities, fair value		254,800,000
Other-than-temporary impairments related to these investments		$ 0
Minimum
Investment [Line Items]
Available-for-sale securities maturity range	3 months
Maximum
Investment [Line Items]
Available-for-sale securities maturity range	1 year

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