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Stock-Based Incentive Plan - Aggregated Assumptions Used to Estimate Fair Value of Options Granted (Detail) - $ / shares
6 Months Ended
Jun. 30, 2017
Jun. 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   1.89%
Expected dividend yield 2.84%  
Expected dividend yield - weighted average   4.07%
Expected volatility 29.75%  
Expected volatility - weighted average   46.13%
Fair value of options $ 6.72 $ 6.26
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected option life in years   7 years 10 months 24 days
Risk free interest rate   1.81%
Expected dividend yield   4.07%
Expected volatility   46.13%
Fair value of options   $ 5.98
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