v3.19.3

Partners' Capital and Distributions (Schedule of Capital Units) (Details) | 9 Months Ended
--- | ---
Sep. 30, 2019 director shares
Increase (Decrease) in Partners' Capital [Roll Forward] |
Limited partner units outstanding, beginning balance (in shares) | 228,195,160
Limited partner units outstanding, ending balance (in shares) | 228,403,428
Management [Member] |
Increase (Decrease) in Partners' Capital [Roll Forward] |
During 2019–Other (in shares) | 199,792
Director [Member] |
Increase (Decrease) in Partners' Capital [Roll Forward] |
During 2019–Other (in shares) | 8,476
Partners' Capital Account, Unit-based Payment Arrangement, Number Of Independent Directors | director | 4

X
- Definition Partners' Capital Account, Unit-based Payment Arrangement, Number Of Independent Directors + References No definition available. + Details
Name: | mmp_PartnersCapitalAccountUnitbasedPaymentArrangementNumberOfIndependentDirectors
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_IncreaseDecreaseInPartnersCapitalRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of limited partner units outstanding. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 5 -Subparagraph (SAB Topic 4.F) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70434-108055 + Details
Name: | us-gaap_LimitedPartnersCapitalAccountUnitsOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of units issued for unit-based payment arrangement issued by partnership. Partner includes, but is not limited to, general, limited and preferred partners. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 272 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055 + Details
Name: | us-gaap_PartnersCapitalAccountUnitsUnitbasedCompensation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_TitleOfIndividualAxis=srt_ManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_TitleOfIndividualAxis=srt_DirectorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |