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Cash, Cash Equivalents, Restricted Cash and Marketable Securities - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Realized gains or losses from the sale of marketable securities	$ 0	$ 0

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- References

No definition available.

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- Definition

Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

No definition available.

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