| **Share-Based Payments - Valuation Assumptions of Total Shareholder Return Units (Detail) - Total Shareholder Return Units (TSRU) [Member]**                                                                                                            | 12 Months Ended |
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| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                                                                                                                                      |                 |   |   |   |
| Expected dividend yield                                                                                                                                                                                                                                 | [1]             | 3.19% | 3.18% | 3.45% |
| Risk-free interest rate                                                                                                                                                                                                                                 | [2]             | 1.76% | 1.78% | 1.03% |
| Expected stock price volatility                                                                                                                                                                                                                         | [3]             | 18.41% | 19.76% | 19.68% |
| Contractual term (years)                                                                                                                                                                                                                                |                 | 5 years 10 months 28 days | 5 years 11 months 19 days | 5 years 11 months 24 days |
|                                                                                                                                                                                                                                                         |
| [1]Determined using a constant dividend yield during the expected term of the TSRU.[2]Determined using the interpolated yield on U.S. Treasury zero-coupon issues.[3]Determined using implied volatility, after consideration of historical volatility. |

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| - Definition
The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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| - Definition
The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
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| - Definition
The risk-free interest rate assumption that is used in valuing an option on its own shares.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| - Definition
Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2
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