Share-Based Payments - Valuation Assumptions of Total Shareholder Return Units (Detail) - Total Shareholder Return Units (TSRUs) [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] | 2.60% | 1.98% | 1.31% | ||||||
| Expected stock price volatility | [3] | 20.00% | 18.39% | 21.64% | ||||||
| Contractual term (years) | 5 years 1 month 13 days | 5 years 1 month 10 days | 5 years 1 month 13 days | |||||||
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