v3.19.1

Partners' Capital and Distributions (Narrative) (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2019 USD ($)
Partners' Capital [Abstract] |
Amount Authorized Under Equity Distribution Agreement | $ 750.0

X
- Definition Amount Authorized Under Equity Distribution Agreement + References No definition available. + Details
Name: | mmp_AmountAuthorizedUnderEquityDistributionAgreement
Namespace Prefix: | mmp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_PartnersCapitalAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration