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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Payment Arrangement [Abstract]      
Risk-free interest rate (percent) 2.63% 2.65% 2.31%
Expected option life (years) 7 years 7 years 7 years
Expected volatility (percent) 21.00% 21.00% 21.00%
Expected dividend yield (percent) 2.70% 2.19% 2.07%
Weighted average fair value of an option share granted during the year (usd per share) $ 4.47 $ 5.44 $ 5.62
Intrinsic value of options exercised during the year (shares) $ 2,994 $ 8,513 $ 7,882