Share Based Compensation Arrangement by Share Based Payment Award, Fair Value Assumptions, 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.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Stock Option Plan - Stock Options Issued (Detail)	12 Months Ended
Stock Option Plan - Stock Options Issued (Detail)	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%