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Share-Based Compensation - Stock Options Assumptions Table (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Weighted-average grant-date fair value $ 14.77 $ 15.92 $ 18.18
Assumptions      
Expected option life—years 4 years 9 months 4 years 1 month 6 days 4 years 10 months 24 days
Volatility 33.60% 38.20% 32.40%
Risk-free interest rate 2.00% 1.30% 1.40%
Dividend yield 0.40% 0.60% 1.40%