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Schedule of Fair Value Other Financial Instruments Measured (Detail) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets $ 423,131 $ 399,322
Quoted Prices In Active Markets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets 423,131 399,322
Cash Equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets 423,131 399,322
Cash Equivalents | Quoted Prices In Active Markets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of assets $ 423,131 $ 399,322