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Summary of Employee Stock Purchase Plan Activity (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares purchased by employees 89,978  
Aggregate cost to employees $ 5,677  
Expense recognized by the company 15,179 $ 15,336
Company expense on ESPP discount    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expense recognized by the company $ 1,002 $ 893