v3.20.1

Revenue from Contract with Customers (Performance Obligations) (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 3,177,657
Refined Products Segment [Member]
Segment Reporting Information [Line Items]
Revenue, Remaining Performance Obligation, Amount	2,058,298
Crude Oil Pipeline and Terminals Segment [Member]
Segment Reporting Information [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,119,359

X

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130556-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_SegmentReportingInformationLineItems
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:

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mmp_CrudeOilPipelineandTerminalsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: