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Incentive Plan - Summary of Fair Value of Stock Options Granted (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Risk-free interest rate 1.72% 2.29% 1.47%
Expected option life 7 years 7 years 7 years
Expected volatility 28.00% 29.00% 29.00%
Expected dividend yield 2.99% 2.71% 3.27%
Weighted average fair value of an option share granted during the year $ 3.58 $ 4.17 $ 3.01
Intrinsic value of options exercised during the year $ 136 $ 131 $ 11