v3.20.2

Fair Value Fair Value (Narrative) (Details) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 Terminal | Jun. 30, 2020 bbl
Derivative [Line Items] |  |
Number Of Marine Terminals Sold | Terminal | 3 |
Energy Related Derivative [Member] |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount | bbl |  | 30,000

X
- Definition Number Of Marine Terminals Sold + References No definition available. + Details
Name: | mmp_NumberOfMarineTerminalsSold
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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
- Definition Nominal number of units used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959 + Details
Name: | us-gaap_DerivativeNonmonetaryNotionalAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EnergyRelatedDerivativeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |