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Incentive Plans (Narrative) (Details) | 12 Months Ended | 76 Months Ended |
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Dec. 31, 2020 USD ($) $ / shares peerCompanies shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2019 shares | Feb. 12, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares granted (in shares) | 79,200 |  |  |  |
Unrecognized compensation cost related to stock option awards | $ | $ 18,100,000 |  |  |  |
Unrecognized compensation cost related to stock option awards, weighted average period for recognition (in years) | 1 year 8 months 12 days |  |  |  |
Total income tax benefits realized from tax deductions related to stock option exercises under share-based payment arrangements | $ | $ 700,000 | $ 100,000 | $ 2,100,000 |  |
2013 Long-Term Incentive Plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Maximum number of shares authorized for incentive plan (in shares) |  |  |  |  | 5,500,000
Maximum number of shares per employee (in shares) | 1,000,000 |  |  |  |
Maximum amount payable | $ | $ 5,000,000.0 |  |  |  |
Shares granted (in shares) |  |  |  | 2,491,559 |
Shares available for grant (in shares) | 3,008,441 |  |  |  |
2013 Stock Plan For Non-Employee Directors |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Maximum number of shares authorized for incentive plan (in shares) | 500,000 |  |  |  |
Shares available for grant (in shares) | 366,570 |  |  |  |
Restricted stock units issued (in shares) |  |  |  | 133,430 |
2013 Stock Plan For Non-Employee Directors | Restricted Stock Units |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Restricted stock units issued (in shares) | 9,801 |  |  |  |
Award vesting period (in years) | 3 years |  |  |  |
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares | $ 105.33 |  |  |  |
MUSA 2013 Plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Restricted stock units issued (in shares) | 55,734 |  |  |  |
Option term (in years) | 7 years |  |  |  |
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares | $ 90.09 |  |  |  |
MUSA 2013 Plan | Return On Average Capital Employed Performance Units |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Award vesting period (in years) | 3 years |  |  |  |
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares | $ 106.72 |  |  |  |
MUSA 2013 Plan | Total Shareholder Return Performance Units |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Award vesting period (in years) | 3 years |  |  |  |
Number of companies in total shareholder return peer comparison group | peerCompanies | 18 |  |  |  |
Restricted stock units issued, weighted average grant date fair value (in dollars per share) | $ / shares | $ 142.07 |  |  |  |
MUSA 2013 Plan | Restricted Stock Units |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Award vesting period (in years) | 3 years |  |  |  |

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- Definition Amount of cost not yet recognized for nonvested award under share-based payment arrangement. + 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 (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Weighted-average period over which cost not yet recognized is expected to be recognized for 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 (i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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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). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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