Stock-Based Compensation - Stock Option Transactions Under Various Stock-Based Employee Compensation Plans (Detail) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, Beginning Balance, Shares	608,375
Granted, Shares	63,500
Exercised, Shares	(61,949)	(340,684)	(361,618)
Expired, Shares	(11,000)
Outstanding, Ending Balance, Shares	598,926	608,375
Exercisable, Ending Balance, Shares	422,626
Outstanding, Beginning Balance, Weighted Average Exercise Price	$ 57.97
Granted, Weighted Average Exercise Price	71.91
Exercised, Weighted-Average Exercise Price	50.66
Expired, Weighted Average Exercise Price	50.81
Outstanding, Ending Balance, Weighted Average Exercise Price	60.34	$ 57.97
Exercisable, Ending Balance, Weighted Average Exercise Price	$ 55.06
Outstanding, Ending balance, Weighted Average Remaining Contractual Term	5 years 5 months 1 day
Exercisable, Ending Balance, Weighted Average Remaining Contractual Term	4 years 3 months

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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