Quarterly report pursuant to Section 13 or 15(d)

Investments in Marketable Securities - Amortized Cost and Fair Value of Marketable Securities, Available for Sale Securities (Detail)

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Investments in Marketable Securities - Amortized Cost and Fair Value of Marketable Securities, Available for Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities continuous unrealized loss position for less than 12 months, unrealized loss $ (172) $ (491)
Available for sale securities continuous unrealized loss position for 12 months or longer, unrealized loss (111) (37)
Available for sale securities continuous unrealized loss position for less than 12 months, fair value 72,509 86,105
Available for sale securities continuous unrealized loss position for 12 months or longer, fair value $ 21,354 $ 721