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Stock-Based Incentive Plan - Aggregated Assumptions Used to Estimate Fair Value of Options Granted (Detail) - $ / shares
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected option life in years 6 years 10 months 24 days  
Expected option life in years - weighted average   7 years 10 months 24 days
Risk free interest rate 2.30%  
Risk free interest rate - weighted average   1.87%
Expected dividend yield 2.84%  
Expected dividend yield - weighted average   4.06%
Expected volatility 29.75%  
Expected volatility - weighted average   45.82%
Fair value of options $ 6.72 $ 6.23
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