v3.19.3

Partners' Capital and Distributions (Narrative) (Details) $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2019 USD ($) shares
Partners' Capital [Abstract] |
Amount Authorized Under Equity Distribution Agreement | $ | $ 750.0
Equity Distribution Agreement, Units Issued (in shares) | shares | 0

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

X
- Definition Equity Distribution Agreement, Units Issued + References No definition available. + Details
Name: | mmp_EquityDistributionAgreementUnitsIssued
Namespace Prefix: | mmp_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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