Stock Based Compensation - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amortization of non-vested stock grants and performance-based awards	$ 1,453,000	$ 1,124,000
Stock options exercised by employees and directors	0	0
Number of shares of non-vested stock that vested	9,307	10,745
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option plan, share based compensation, vesting period (in years)	3 years
Aggregate fair value	$ 800,000
Performance Shares [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted under performance plan	19,500
Employees and Directors [Member] \| Non-Vested Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of non-vested stock issued	1,844
Aggregate fair value	$ 155,100
Employees and Directors [Member] \| Non-Vested Shares [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option plan, share based compensation, vesting period (in years)	1 year
Fair market value of the non-vested stock on the date of grant	$ 84.06
Employees and Directors [Member] \| Non-Vested Shares [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option plan, share based compensation, vesting period (in years)	5 years
Fair market value of the non-vested stock on the date of grant	$ 84.33

Amortization of non vested stock grants and performance based awards.

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options grants in period aggregate grant date fair value.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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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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Number of shares issued under share-based payment arrangement.

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

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