v3.20.1

Revenue from Contract with Customers (Narrative) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 USD ($)
Disaggregation of Revenue [Line Items] |
Deferred Revenue, Revenue Recognized | $ 91.4
Average Delivery Point of Barrels in Pipeline System | 50.00%
Refined Products Segment [Member] |
Disaggregation of Revenue [Line Items] |
Revenue, Shipments Regulated By FERC, Percentage | 40.00%
Revenue, Markets Regulated By State Or FERC, Percentage | 60.00%

X
- Definition Average Delivery Point of Barrels in Pipeline System + References No definition available. + Details
Name: | mmp_AverageDeliveryPointofBarrelsinPipelineSystem
Namespace Prefix: | mmp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Revenue, Markets Regulated By State Or FERC, Percentage + References No definition available. + Details
Name: | mmp_RevenueMarketsRegulatedByStateOrFERCPercentage
Namespace Prefix: | mmp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Revenue, Shipments Regulated By FERC, Percentage + References No definition available. + Details
Name: | mmp_RevenueShipmentsRegulatedByFERCPercentage
Namespace Prefix: | mmp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of revenue recognized that was previously reported as deferred or unearned revenue. + References No definition available. + Details
Name: | us-gaap_DeferredRevenueRevenueRecognized1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_DisaggregationOfRevenueLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mmp_RefinedProductsSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |