v3.19.3

Commitments and Contingencies (Details) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Oct. 04, 2017 facility
Loss Contingencies [Line Items] |  |  |  |  |  |
Liabilities recognized for estimated environmental costs | $ 16.6 |  | $ 16.6 |  | $ 20.5 |
Environmental expenses | 0.8 | $ 5.5 | 4.2 | $ 10.8 |  |
Receivables from insurance carriers related to environmental matters | 3.0 |  | 3.0 |  | 4.1 |
Accounts Receivable [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Receivables from insurance carriers related to environmental matters | 1.4 |  | 1.4 |  | 2.4 |
Other Noncurrent Assets [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Receivables from insurance carriers related to environmental matters | $ 1.6 |  | $ 1.6 |  | $ 1.7 |
Powder Springs Logistics, LLC [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Interest in equity method investment | 50.00% |  | 50.00% |  |  |
Powder Springs Logistics, LLC [Member] | Guarantee of Indebtedness of Others [Member] | Equity Method Investee [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Interest in equity method investment | 50.00% |  | 50.00% |  |  |
Guarantor Obligations, Maximum Exposure, Undiscounted | $ 25.0 |  | $ 25.0 |  |  |
Guarantees, Fair Value Disclosure | $ 0.4 |  | $ 0.4 |  |  |
Pending Litigation [Member] | Unfavorable Regulatory Action [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Loss Contingency, Alleged Patent Infringement, Number Of Facilities | facility |  |  |  |  |  | 9

X
- Definition Loss Contingency, Alleged Patent Infringement, Number Of Facilities + References No definition available. + Details
Name: | mmp_LossContingencyAllegedPatentInfringementNumberOfFacilities
Namespace Prefix: | mmp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 + Details
Name: | us-gaap_AccrualForEnvironmentalLossContingencies
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=109237650&loc=d3e13064-110858 Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859 + Details
Name: | us-gaap_EnvironmentalRemediationExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair value portion of guarantee, including, but not limited to, financial standby letter of credit, fair value guarantee, guarantee of collection of scheduled contractual cash flows from financial assets, and contracts that contingently require the guarantor to make payments or perform services. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12201-110248 + Details
Name: | us-gaap_GuaranteesFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying amount as of the balance sheet date of known and reasonably estimable amounts of recoveries from third-parties pertaining to an entity's environmental remediation obligations. An entity's balance sheet may include several assets that relate to an environmental remediation obligation. Among them are the following: a. Receivables from other potentially responsible parties that are not providing initial funding; b. anticipated recoveries from insurers; and c. anticipated recoveries from prior owners as a result of indemnification agreements. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237650&loc=d3e13022-110858 + Details
Name: | us-gaap_RecordedThirdPartyEnvironmentalRecoveriesReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_AccountsReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mmp_PowderSpringsLogisticsLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_GuaranteeOfIndebtednessOfOthersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_EquityMethodInvesteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_UnfavorableRegulatoryActionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |