v3.19.3.a.u2

Long-Term Incentive Plan (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2019
Share-based Payment Arrangement [Abstract] |
Changes In Non-Vested Unit Awards | The amounts below do not include adjustments for above-target or below-target performance.
|  | Performance-Based Awards |  | Time-Based Awards |  | Total Awards
|  | Number of Unit Awards |  | Weighted-Average Fair Value |  | Number of Unit Awards |  | Weighted-Average Fair Value |  | Number of Unit Awards |  | Weighted-Average Fair Value
Non-vested units - 1/1/2019 |  | 390,015 |  |  | $ | 77.66 |  |  | 111,388 |  |  | $ | 73.09 |  |  | 501,403 |  |  | $ | 76.65 |
Units granted during 2019 |  | 182,834 |  |  | $ | 63.65 |  |  | 195,031 |  |  | $ | 62.91 |  |  | 377,865 |  |  | $ | 63.27 |
Units vested during 2019 |  | (173,842 | ) |  | $ | 82.31 |  |  | (30,958 | ) |  | $ | 78.05 |  |  | (204,800 | ) |  | $ | 81.67 |
Units forfeited during 2019 |  | (19,103 | ) |  | $ | 70.69 |  |  | (15,145 | ) |  | $ | 65.18 |  |  | (34,248 | ) |  | $ | 68.25 |
Non-vested units - 12/31/19 |  | 379,904 |  |  | $ | 69.14 |  |  | 260,316 |  |  | $ | 63.92 |  |  | 640,220 |  |  | $ | 67.02 |
Total Non-Vested Unit Awards | The table below summarizes the total non-vested unit awards outstanding adjusted for estimated amounts of above-target financial performance to determine the total number of unit awards included in our total equity-based liability accrual.
Grant Date |  | Non-Vested Unit Awards |  | Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results |  | Total Unit Award Accrual |  | Vesting Date |  | Unrecognized Compensation Expense(a) (in millions) |
Performance-Based Awards: |  |  |  |  |  |  |  |  |  |  |
2018 Awards |  | 205,568 |  |  | — |  |  | 205,568 |  |  | 12/31/2020 |  | $ | 4.9 |  |
2019 Awards |  | 174,336 |  |  | — |  |  | 174,336 |  |  | 12/31/2021 |  | 7.3 |  |
Time-Based Awards: |  |  |  |  |  |  |  |  |  |  |
2020 Vesting Date |  | 76,820 |  |  | — |  |  | 76,820 |  |  | 12/31/2020 |  | 1.9 |  |
2021 Vesting Date |  | 180,162 |  |  | — |  |  | 180,162 |  |  | 12/31/2021 |  | 7.5 |  |
2022 Vesting Date |  | 3,334 |  |  | — |  |  | 3,334 |  |  | 12/31/2022 |  | 0.2 |  |
Total |  | 640,220 |  |  | — |  |  | 640,220 |  |  |  |  | $ | 21.8 |  |
(a) Unrecognized compensation expense will be recognized over the remaining vesting period of the awards.
Weighted-Average Grant Date Fair Values | The weighted-average fair value of awards granted during 2017, 2018 and 2019 was as follows:
|  | Performance-Based Awards |  | Time-Based Awards
|  | Number of Unit Awards |  | Weighted-Average Fair Value |  | Number of Unit Awards |  | Weighted-Average Fair Value
Units granted during 2017 |  | 189,544 |  |  | $ | 82.34 |  |  | 30,604 |  |  | $ | 79.10 |
Units granted during 2018 |  | 218,923 |  |  | $ | 73.80 |  |  | 83,564 |  |  | $ | 71.03 |
Units granted during 2019 |  | 182,834 |  |  | $ | 63.65 |  |  | 195,031 |  |  | $ | 62.91 |
Vested Unit Awards | The table below sets forth the numbers and values of units that vested in each of the three years ended December 31, 2019. The vested common units include adjustments for above-target financial and market performance.
Vesting Date |  | Vested Common Units |  | Fair Value of Unit Awards on Vesting Date (in millions) |  | Intrinsic Value of Unit Awards on Vesting Date (in millions)
12/31/2017 |  | 266,028 |  | $19.9 |  | $18.9
12/31/2018 |  | 317,037 |  | $22.1 |  | $18.1
12/31/2019 |  | 436,629 |  | $31.0 |  | $27.5
Cash Flow Effects Of Long Term Incentive Plan Settlements [Table Text Block] | The difference between the common units issued to the participants and the total number of unit awards vested primarily represents the tax withholdings associated with the award settlement, which we pay in cash.
Settlement Date |  | Number of Common Units Issued, Net of Tax Withholdings |  | Tax Withholdings and Other Cash Payments (in millions) |  | Employer Taxes (in millions) |  | Total Cash Payments (in millions)
January 2017 |  | 216,679 |  | $13.9 |  | $1.2 |  | $15.1
January 2018 |  | 168,913 |  | $9.3 |  | $1.1 |  | $10.4
January 2019 |  | 199,792 |  | $9.8 |  | $0.9 |  | $10.7
Equity-Based Incentive Compensation Expense | Equity-based incentive compensation expense for 2017, 2018 and 2019, primarily recorded as G&A expense on our consolidated statements of income, was as follows (in thousands):
|  | Year Ended December 31,
|  | 2017 |  | 2018 |  | 2019
Performance awards |  | $ | 17,823 |  |  | $ | 28,728 |  |  | $ | 17,920 |
Time-based awards |  | 2,818 |  |  | 3,325 |  |  | 6,092 |
Total |  | $ | 20,641 |  |  | $ | 32,053 |  |  | $ | 24,012 |

X
- Definition Cash Flow Effects Of Long Term Incentive Plan Settlements [Table Text Block] + References No definition available. + Details
Name: | mmp_CashFlowEffectsOfLongTermIncentivePlanSettlementsTableTextBlock
Namespace Prefix: | mmp_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of cost recognized for award under share-based payment arrangement by plan. Includes, but is not limited to, related tax benefit. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ScheduleOfCompensationCostForShareBasedPaymentArrangementsAllocationOfShareBasedCompensationCostsByPlanTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the changes in outstanding nonvested performance-based units. + 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) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ScheduleOfNonvestedPerformanceBasedUnitsActivityTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of number, weighted-average exercise price or conversion ratio, aggregate intrinsic value, and weighted-average remaining contractual term for outstanding performance-based awards that are fully vested and expected to vest. Includes, but is not limited to, unvested awards for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceBasedUnitsVestedAndExpectedToVestTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of cost not yet recognized and weighted-average period over which cost is expected to be 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=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ScheduleOfUnrecognizedCompensationCostNonvestedAwardsTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the weighted-average grant-date fair value of equity options or other equity instruments granted during the year. + 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValueTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration