Share-Based Payments - Valuation Assumptions of Stock Options (Detail) - Employee Stock Option [Member] |
12 Months Ended | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||||||||||
| Expected dividend yield | [1] | 3.73% | 3.69% | 3.85% | ||||||||
| Risk-free interest rate | [2] | 2.85% | 2.23% | 1.55% | ||||||||
| Expected stock price volatility | [3] | 20.02% | 18.39% | 21.64% | ||||||||
| Expected term (years) | [4] | 6 years 9 months | 6 years 9 months | 6 years 9 months | ||||||||
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