Stock Plans (Schedule of Non-Vested Restricted Stock Units Activity) (Details) - Restricted Stock [Member] - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested balance at December 31	27,161	29,960	20,704
Granted	18,311	12,901	19,250
Forfeited		(126)
Vested	(15,395)	(15,574)	(9,994)
Non-vested balance at December 31	30,077	27,161	29,960
Weighted-average grant date fair value, Non-vested balance at December 31	$ 49.48	$ 38.99	$ 23.59
Weighted-average grant date fair value, Granted	55.00	54.70	47.44
Weighted-average grant date fair value, Forfeited		54.69
Weighted-average grant date fair value, Vested	48.47	33.81	23.69
Weighted-average grant date fair value, Non-vested balance at December 31	$ 53.36	$ 49.48	$ 38.99
Director Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested balance at December 31	11,200	12,600	8,400
Granted	6,000	5,600	8,400
Vested	(7,798)	(7,000)	(4,200)
Non-vested balance at December 31	9,402	11,200	12,600
Weighted-average grant date fair value, Granted	$ 56.63	$ 54.69	$ 49.50
Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested balance at December 31	15,961	17,360	12,304
Granted	12,311	7,301	10,850
Forfeited		(126)
Vested	(7,597)	(8,574)	(5,794)
Non-vested balance at December 31	20,675	15,961	17,360

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

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)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

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-Publisher FASB
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-Topic 718
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-Section 50
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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 non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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.

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-Publisher FASB
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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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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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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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