v3.20.2

Derivative Financial Instruments (Derivative Gains Included In Accumulated Other Comprehensive Loss (AOCL) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Derivative Gains Included in AOCI [Roll Forward] |  |  |  |
Beginning balance | $ 2,454,738 | $ 2,637,779 | $ 2,715,028 | $ 2,643,434
Net gain (loss) on cash flow hedges | 0 | (14,181) | (10,444) | (25,216)
Reclassification of net loss (gain) on cash flow hedges to income | 896 | 699 | 2,552 | 1,927
Ending balance | 2,388,444 | 2,674,456 | 2,388,444 | 2,674,456
Derivative Losses Included in AOCL |  |  |  |
Derivative Gains Included in AOCI [Roll Forward] |  |  |  |
Beginning balance | (57,748) | (36,287) | (48,960) | (26,480)
Ending balance | $ (56,852) | $ (49,769) | $ (56,852) | $ (49,769)

X
- Definition Derivative Gains Included in Accumulated Other Comprehensive Loss [Roll Forward] + References No definition available. + Details
Name: | mmp_DerivativeGainsIncludedInAccumulatedOtherComprehensiveLossRollForward
Namespace Prefix: | mmp_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after tax and before reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 + Details
Name: | us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAfterTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount, after tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e689-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580 + Details
Name: | us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationAfterTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of ownership interest of different classes of partners in limited partnership. + 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=120397183&loc=d3e187085-122770 + Details
Name: | us-gaap_PartnersCapital
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_PartnerCapitalComponentsAxis=us-gaap_AccumulatedGainLossNetCashFlowHedgeParentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |