Incentive Plans (Summary of Restricted Stock Unit Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended	76 Months Ended
Dec. 31, 2020	Dec. 31, 2019
MUSA 2013 Plan
Number of units
Beginning balance (in shares)	198,915
Granted (in shares)	55,734
Vested and issued (in shares)	(59,324)
Forfeited (in shares)	(12,530)
Ending balance (in shares)	182,795	198,915
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share)	$ 70.58
Granted (in dollars per share)	90.09
Vested and issued (in dollars per share)	66.31
Forfeited (in dollars per share)	78.43
Ending balance (in dollars per share)	$ 77.38	$ 70.58
Total fair value vested	$ 6.4
Total fair value,outstanding	$ 23.9
MUSA 2013 Plan \| Performance Units
Number of units
Beginning balance (in shares)	131,200
Granted (in shares)	64,290
Vested and issued (in shares)	(65,745)
Forfeited (in shares)	(405)
Ending balance (in shares)	129,340	131,200
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share)	$ 82.98
Granted (in dollars per share)	122.95
Vested and issued (in dollars per share)	85.04
Forfeited (in dollars per share)	65.75
Ending balance (in dollars per share)	$ 97.01	$ 82.98
Total fair value vested	$ 7.0
Total fair value,outstanding	$ 16.9
2013 Stock Plan For Non-Employee Directors
Number of units
Granted (in shares)		133,430
2013 Stock Plan For Non-Employee Directors \| Restricted Stock Units
Number of units
Beginning balance (in shares)	33,607
Granted (in shares)	9,801
Vested and issued (in shares)	(12,404)
Ending balance (in shares)	31,004	33,607
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share)	$ 70.68
Granted (in dollars per share)	105.33
Vested and issued (in dollars per share)	66.01
Ending balance (in dollars per share)	$ 83.31	$ 70.68
Total fair value vested	$ 1.3
Total fair value,outstanding	$ 4.1

Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Fair Value

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- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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.

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- Definition

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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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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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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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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