v3.19.3.a.u2

Debt (Narrative) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Feb. 11, 2019 USD ($)
Debt Instrument [Line Items] |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months | $ 0 |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Two | 550,000,000 |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Three | 0 |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Four | 0 |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Five | 0 |  |  |
Long-term Debt, Maturities, Repayments of Principal after Year Five | 4,200,000,000 |  |  |
Debt Instrument, Face Amount | 4,750,000,000 | $ 4,300,000,000 |  |
Payment for Debt Extinguishment or Debt Prepayment Cost | 8,270,000 | 0 | $ 0 |
Proceeds from Issuance of Senior Long-term Debt | 996,405,000 | 0 | $ 496,705,000 |
Long-term Debt | $ 4,706,075,000 | $ 4,270,869,000 |  |
Long-term Debt, Weighted Average Interest Rate, over Time | 4.60% | 4.80% | 4.80% |
Cash Payments for Interest | $ 217,100,000 | $ 227,800,000 | $ 206,200,000 |
Revolving Credit Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 1,000,000,000.0 |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.125% |  |  |
Letters of Credit Outstanding, Amount | $ 3,500,000 | $ 6,800,000 |  |
Debt Instrument, Consolidated Debt to EBITDA Ratio | 5.0 |  |  |
Revolving Credit Facility [Member] | Minimum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 1.00% |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.10% |  |  |
Revolving Credit Facility [Member] | Maximum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | 1.625% |  |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.275% |  |  |
Commercial Paper [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 1,000,000,000.0 |  |  |
Long-term Debt, Weighted Average Interest Rate, over Time | 2.60% | 2.30% |  |
Commercial Paper [Member] | Maximum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Term | 397 days |  |  |
3.95% Notes due 2050 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount | $ 500,000,000.0 | $ 0 |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.95% |  |  |
Debt Instrument, Notes At Price | 99.91% |  |  |
Proceeds from Issuance of Senior Long-term Debt | $ 494,400,000 |  |  |
4.20% Notes Due 2047 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount | $ 500,000,000 | 500,000,000 |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.20% |  |  |
4.85% Notes Due 2049 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount | $ 500,000,000.0 | 0 |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.85% |  |  |
Payment for Debt Extinguishment or Debt Prepayment Cost | $ 8,300,000 |  |  |
Debt Instrument, Notes At Price | 99.371% |  |  |
Proceeds from Issuance of Senior Long-term Debt | $ 491,500,000 |  |  |
6.55% Notes Due 2019 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount | $ 0 | 550,000,000 |  | $ 550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage | 6.55% |  |  | 6.55%
Line of Credit [Member] | Revolving Credit Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term Debt | $ 0 | $ 0 |  |

X
- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
Name: | mmp_DebtInstrumentConsolidatedDebtToEbitdaRatio
Namespace Prefix: | mmp_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt Instrument, Notes At Price, Percentage Of Par + References No definition available. + Details
Name: | mmp_DebtInstrumentNotesAtPricePercentageOfPar
Namespace Prefix: | mmp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400 + Details
Name: | us-gaap_DebtInstrumentFaceAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentTerm
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cash paid for interest, including, but not limited to, capitalized interest and payment to settle zero-coupon bond attributable to accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount; classified as operating and investing activities. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4297-108586 + Details
Name: | us-gaap_InterestPaid
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility. + References No definition available. + Details
Name: | us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04.(a)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Weighted average interest rate of long-term debt outstanding calculated over time. + References No definition available. + Details
Name: | us-gaap_LongTermDebtWeightedAverageInterestRateOverTime
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cash outflow for cost from early extinguishment and prepayment of debt. Includes, but is not limited to, third-party cost, premium paid, and other fee paid to lender directly for debt extinguishment or debt prepayment. Excludes accrued interest. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
Name: | us-gaap_PaymentsOfDebtExtinguishmentCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_CommercialPaperMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=mmp_ThreePointNineFivePercentageNotesDueTwoThousandFiftyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=mmp_FourPointTwoZeroPercentageNotesDueTwoThousandFortySevenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=mmp_FourPointEightFivePercentageNotesDueTwoThousandFortyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=mmp_SixPointFiveFivePercentageNotesDueTwoThousandNinteenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |