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Stock-Based Compensation (Valuation and Expense Information) - Additional Information (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Stock options, granted 0    
Weighted-average estimated value   $ 5.22 $ 4.28
Contractual term 10 years    
Weighted average grant date fair value, PSUs granted $ 21.69    
Performance Stock Units (PSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value, PSUs granted $ 24.17 $ 23.50 $ 17.64