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Stock Option Plan - Stock Options Issued (Detail) | 12 Months Ended
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Dec. 31, 2020 $ / shares | Dec. 31, 2019 $ / shares | Dec. 31, 2018 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 49.19 | $ 44.53 | $ 42.98
Directors | May 11, 2018 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 49.46 |  |
Volatility | 0.192 |  |
Expected life (years) | 5 years |  |
Assumed yield | 3.70% |  |
Risk-free rate | 2.84% |  |
Directors | May 3, 2019 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 55.71 |  |
Volatility | 0.236 |  |
Expected life (years) | 5 years |  |
Assumed yield | 3.75% |  |
Risk-free rate | 2.33% |  |
Directors | April, 24 2020 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 50.00 |  |
Volatility | 0.258 |  |
Expected life (years) | 5 years |  |
Assumed yield | 3.80% |  |
Risk-free rate | 0.36% |  |
Officers | May 11, 2018 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 49.46 |  |
Volatility | 0.177 |  |
Expected life (years) | 7 years |  |
Assumed yield | 3.75% |  |
Risk-free rate | 2.94% |  |
Officers | May 3, 2019 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 55.71 |  |
Volatility | 0.206 |  |
Expected life (years) | 7 years |  |
Assumed yield | 3.80% |  |
Risk-free rate | 2.43% |  |
Officers | April, 24 2020 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price (in usd per share) | $ 50.00 |  |
Volatility | 0.240 |  |
Expected life (years) | 7 years |  |
Assumed yield | 3.85% |  |
Risk-free rate | 0.51% |  |

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- Definition Share Based Compensation Arrangement by Share Based Payment Award, Fair Value Assumptions, Volatility + References No definition available. + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=120381028&loc=d3e5070-113901 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=120381028&loc=d3e5070-113901 + Details
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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. + Details
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- Definition Weighted average price at which option holders acquired shares when converting their stock options into shares. + References No definition available. + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -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=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=115993241&loc=d3e301413-122809 + Details
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