Stock Compensation (Schedule Of Weighted-Average Assumptions Used To Determine The Fair Value Of Options) (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options	5 years	5 years	5 years
Expected stock price volatility	0.35%	0.40%	0.40%
Risk-free interest rate	1.41%	2.54%	2.64%
ESPP Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options	6 months	6 months	6 months
Expected stock price volatility	0.52%	0.39%	0.34%
Risk-free interest rate	1.00%	2.35%	1.73%

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

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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.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ 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 (f)(2)(i)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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