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Available-for-sale Securities and Fair Value on Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Other-than-temporary impairment losses	$ 0	$ 0	$ 0
Remaining investment in common stock	136,853,000	115,555,000
Gross realized losses from the sale or maturity of available-for-sale investments	$ 0	$ 0	0
Aduro Biotech Inc
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from the sale of available-for-sale investments	$ 0	$ 3,500,000	$ 800,000
Sale of common stock available-for-sale investments	0	346,700	50,000
Remaining investment in common stock	0	0

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Sale of common stock in marketable equity investment.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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