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Share-Based Compensation - Unrecognized Compensation Expense and Weighted Average Period (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Stock Option [Member]  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Unrecognized expense $ 9,064
Weighted average remaining years 1 year 8 months 9 days
Performance Share Units [Member]  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Unrecognized expense $ 1,176
Weighted average remaining years 1 year 3 months 22 days