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Stock-based Compensation Plans - Discounted Stock Purchase Plan Assumptions and Resulting Weighted Average Fair Value (Detail) - DSPP - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Dividend yield 2.70% 2.70% 2.30%
Expected volatility 19.00% 25.00% 27.00%
Risk-free interest rate 1.00% 0.50% 0.20%
Weighted-average fair value per share $ 9.46 $ 10.37 $ 12.45