Stock Plans (Summary of Weighted-Average Fair-Value and Vesting of Restricted Stock Grants) (Details) - Restricted Stock [Member] - $ / shares	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	17,811	12,901
Weighted-average grant date fair value	$ 55.59	$ 54.70
Director Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	6,000	5,600
Weighted-average grant date fair value	$ 56.63	$ 54.69
Vesting period	3 years	3 years
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%
Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Award Date One [Member] \| Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	11,761	7,251
Weighted-average grant date fair value	$ 55.06	$ 54.69
Vesting period		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%
Award Date Two [Member] \| Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	50	50
Weighted-average grant date fair value	$ 57.62	$ 58.08
Vesting period	1 year	1 year

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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-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).

+ 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
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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
-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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Percentage of vesting of award under share-based payment arrangement.

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