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Incentive Plans (Summary of Valuation Assumptions) (Details) - MUSA 2013 Plan - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value per option grant (in dollars per share)	$ 28.28	$ 20.48	$ 17.32
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	28.10%	26.80%	27.00%
Risk-free interest rate	1.50%	2.50%	2.40%
Expected life (in years)	4 years 8 months 12 days	4 years 6 months	3 years 10 months 24 days
Stock price at valuation date (usd per share)	$ 106.72	$ 76.15	$ 71.00

X

- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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- Definition

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.

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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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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Price of a single share of a number of saleable stocks of a company.

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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.

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