v3.10.0.1

Organization and Description of Business (Narrative) (Details) bbl in Millions | Dec. 31, 2018 Terminal bbl mi
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Refined Products Segment [Member] |
Organization and Description of Business [Line Items] |
Number of Pipeline Terminals | Terminal | 53
Number of Independent Terminals | Terminal | 25
Refined Products Segment [Member] | Refined Products Transportation Services [Member] |
Organization and Description of Business [Line Items] |
Pipeline, Length | mi | 9,700
Refined Products Segment [Member] | Ammonia Transportation Services [Member] |
Organization and Description of Business [Line Items] |
Pipeline, Length | mi | 1,100
Crude Oil Pipeline and Terminals Segment [Member] |
Organization and Description of Business [Line Items] |
Pipeline, Length | mi | 2,200
Storage Capacity | 33
Contracted Storage | 21
Pipeline, Length - Wholly Owned | mi | 1,000
Storage Capacity - Wholly Owned | 28
Contracted Storage - Wholly Owned | 19
Marine Storage Segment [Member] |
Organization and Description of Business [Line Items] |
Number of Independent Terminals | Terminal | 6
Storage Capacity | 27
Storage Capacity - Wholly Owned | 25
Number of Independent Terminals - Wholly Owned | Terminal | 5

X
- Definition Contracted Storage + References No definition available. + Details
Name: | mmp_ContractedStorage
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Contracted Storage - Wholly Owned + References No definition available. + Details
Name: | mmp_ContractedStorageWhollyOwned
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of PipelineTerminals + References No definition available. + Details
Name: | mmp_NumberOfPipelineTerminals
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Independent Terminals + References No definition available. + Details
Name: | mmp_NumberofIndependentTerminals
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Independent Terminals - Wholly Owned + References No definition available. + Details
Name: | mmp_NumberofIndependentTerminalsWhollyOwned
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition [Line Items] for Organization and Description of Business [Table] + References No definition available. + Details
Name: | mmp_OrganizationAndDescriptionOfBusinessLineItems
Namespace Prefix: | mmp_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Pipeline, length + References No definition available. + Details
Name: | mmp_PipelineLength
Namespace Prefix: | mmp_
Data Type: | num:lengthItemType
Balance Type: | na
Period Type: | instant

X
- Definition Pipeline, Length - Wholly Owned + References No definition available. + Details
Name: | mmp_PipelineLengthWhollyOwned
Namespace Prefix: | mmp_
Data Type: | num:lengthItemType
Balance Type: | na
Period Type: | instant

X
- Definition Storage Capacity + References No definition available. + Details
Name: | mmp_StorageCapacity
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Storage Capacity - Wholly Owned + References No definition available. + Details
Name: | mmp_StorageCapacityWhollyOwned
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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

X
- Details
Name: | srt_ProductOrServiceAxis=mmp_RefinedProductsTransportationServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=mmp_AmmoniaTransportationServicesMember
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: |

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