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Stock-Based Incentive Plan - Aggregated Assumptions Used to Estimate Fair Value of Options Granted (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]      
Expected option life in years     8 years
Expected option life in years - weighted average 6 years 10 months 24 days 7 years 10 months 24 days  
Risk free interest rate     1.64%
Risk free interest rate - weighted average 2.30% 1.87%  
Expected dividend yield     4.32%
Expected dividend yield - weighted average 2.84% 4.06%  
Expected volatility     49.38%
Expected volatility - weighted average 29.75% 45.82%  
Fair value of options $ 6.72 $ 6.23 $ 6.07
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option life in years   7 years 7 months 6 days  
Risk free interest rate   1.38%  
Expected dividend yield   3.80%  
Expected volatility   37.71%  
Fair value of options   $ 5.40  
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected option life in years   8 years 2 months 12 days  
Risk free interest rate   1.89%  
Expected dividend yield   4.19%  
Expected volatility   46.22%  
Fair value of options   $ 6.27