v3.19.2

Derivative Financial Instruments (Schedule Of NYMEX Contracts And Butane Price Swap Purchase Agreements) (Details) bbl in Millions | Jun. 30, 2019 bbl
--- | ---
Economic Hedges [Member] |
Derivative [Line Items] |
Derivative, Nonmonetary Notional Amount | 4.3
Economic Hedges Futures [Member] |
Derivative [Line Items] |
Derivative, Nonmonetary Notional Amount | 1.3

X
- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:decimalItemType
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_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=mmp_EconomicHedgesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=mmp_EconomicHedgesFuturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |