Restricted Stock and Stock Options - Restricted Stock Activity (Detail) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
2012 Incentive Plan [Member] \| Common and Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Nonvested outstanding, Beginning balance	32,528	39,575	60,879
Shares, Granted	23,207	22,713	3,304
Shares, Vested	(40,421)	(29,760)	(24,608)
Shares, Nonvested outstanding, Ending balance	15,314	32,528	39,575
Weighted-Average Grant-Date Fair Value per Share, Beginning balance	$ 54.02	$ 48.93	$ 45.71
Weighted-Average Grant-Date Fair Value per Share, Granted	58.62	61.08	59.16
Weighted-Average Grant-Date Fair Value per Share, Vested	54.31	51.86	44.31
Weighted-Average Grant-Date Fair Value per Share, Ending balance	$ 60.23	$ 54.02	$ 48.93
Non-Employee Director Restricted Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Nonvested outstanding, Beginning balance	4,890	5,761	6,454
Shares, Granted	5,269	4,890	5,761
Shares, Vested	(5,091)	(5,761)	(6,454)
Shares, Nonvested outstanding, Ending balance	5,068	4,890	5,761
Weighted-Average Grant-Date Fair Value per Share, Beginning balance	$ 61.34	$ 60.75	$ 54.20
Weighted-Average Grant-Date Fair Value per Share, Granted	68.88	61.34	60.75
Weighted-Average Grant-Date Fair Value per Share, Vested	61.49	60.75	54.20
Weighted-Average Grant-Date Fair Value per Share, Ending balance	$ 69.03	$ 61.34	$ 60.75

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).

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 (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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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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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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