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Investments - Schedule of Contractual Maturities of Long-Term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]    
Due after one year through five years $ 78,561 $ 70,005
Due after five years through six years 2,121 8,970
Total $ 80,682 $ 78,975