Stock Plans (Summary of Weighted-Average Fair-Value and Vesting of Restricted Stock Grants) (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]
Shares granted	18,311	12,901	19,250
Weighted-average grant date fair value	$ 55.00	$ 54.70	$ 47.44
Director Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	6,000	5,600	8,400
Weighted-average grant date fair value	$ 56.63	$ 54.69	$ 49.50
Award Date One [Member] \| Director Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years	3 years	3 years
Award Date One [Member] \| Director Plan [Member] \| Share-based Compensation Award, Tranche One, Two and Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, percentage per year	33.00%	33.00%	33.00%
Award Date One [Member] \| Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	11,761	7,251	10,800
Weighted-average grant date fair value	$ 55.06	$ 54.69	$ 45.83
Vesting period	3 years	3 years	3 years
Award Date One [Member] \| Omnibus Plan [Member] \| Share-based Compensation Award, Tranche One, Two and Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, percentage per year		33.00%	33.00%
Award Date Two [Member] \| Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	50	50	50
Weighted-average grant date fair value	$ 57.62	$ 58.08	$ 49.50
Vesting period	1 year	1 year	1 year
Award Date Three [Member] \| Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	500
Weighted-average grant date fair value	$ 34.02
Vesting period	3 years
Award Date Three [Member] \| Omnibus Plan [Member] \| Share-based Compensation Award, Tranche One, Two and Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, percentage per year	33.00%

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.

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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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Net number of non-option equity instruments granted to participants.

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-Publisher FASB
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-Topic 718
-SubTopic 10
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