v3.10.0.1

Revenue from Contract with Customers (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Revenue from Contract with Customer [Abstract] |
Deferred Revenue, Revenue Recognized | $ 79.3
Average Delivery Point of Barrels in Pipeline System | 50.00%

X
- Definition Average Delivery Point of Barrels in Pipeline System + References No definition available. + Details
Name: | mmp_AverageDeliveryPointofBarrelsinPipelineSystem
Namespace Prefix: | mmp_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of revenue recognized that was previously reported as deferred or unearned revenue. + References No definition available. + Details
Name: | us-gaap_DeferredRevenueRevenueRecognized1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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