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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 |

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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 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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