| **Stock-based compensation - Fair Value Assumptions of Stock Options (Detail) - Stock Options** | 3 Months Ended         |
|-------------------------------------------------------------------------------------------------|------------------------|
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| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**              |                   |   |
| Expected volatility, minimum                                                                    | 30.40%                 | 30.60% |
| Expected volatility, maximum                                                                    | 31.80%                 | 30.80% |
| Risk-free interest rate, minimum                                                                | 3.79%                  | 1.36% |
| Risk-free interest rate, maximum                                                                | 3.82%                  | 1.47% |
| Minimum                                                                                         |                   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**              |                   |   |
| Expected dividend yield                                                                         | 1.12%                  | 0.76% |
| Expected life of the option (in years)                                                          | 5 years                | 5 years 3 months 18 days |
| Maximum                                                                                         |                   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**              |                   |   |
| Expected dividend yield                                                                         | 1.12%                  | 0.76% |
| Expected life of the option (in years)                                                          | 6 years 1 month 6 days | 6 years 2 months 12 days |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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| - Definition
The estimated measure of the maximum 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
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The estimated measure of the minimum 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
No definition available.
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| - Definition
The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                        |
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| - Definition
The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
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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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
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| - Definition
Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901
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