Quarterly report pursuant to Section 13 or 15(d)

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

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Investments in Marketable Securities - Amortized Cost and Fair Value of Investments in Available for Sale Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities continuous unrealized loss position for less than 12 months, unrealized loss $ (508) $ (158)
Available for sale securities continuous unrealized loss position for 12 months or longer, unrealized loss (282) (236)
Available for sale securities continuous unrealized loss position for less than 12 months, fair value 96,685 63,229
Available for sale securities continuous unrealized loss position for 12 months or longer, fair value $ 28,880 $ 44,961