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Restricted Stock and Stock Options - Restricted Stock Activity (Detail) - $ / shares | 12 Months Ended
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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

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- Definition 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). + 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)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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). + 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)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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 + Details
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- Definition 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 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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)(iii)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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. + 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)(iii)(2) -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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