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Employee Benefit Plans - Significant Assumptions Used in Calculating the Fair Value of the Grants (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Compensation Related Costs [Abstract]      
Weighted-average life 5 years 2 months 12 days 5 years 1 month 6 days 4 years 10 months 25 days
Risk-free interest rate 1.91% 1.43% 1.54%
Dividend yield 2.67% 2.97% 2.91%
Volatility factor 21.47% 23.92% 26.27%
Fair value of the grants $ 6.02 $ 5.09 $ 5.57