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Long-Term Investments - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturities (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Available-for-sale Securities, Debt Maturities [Abstract]  
Due in twenty to forty years, Cost $ 3,000
Due in twenty to forty years, Estimated Fair Value $ 2,525