Effective Ownership Interest, Percentage

No definition available.

Name:	mlp_EffectiveOwnershipInterestPercentage
Namespace Prefix:	mlp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Environmental Impact Statement, Estimated Time To Prepare, Period

+ References

No definition available.

+ Details

Name:	mlp_EnvironmentalImpactStatementEstimatedTimeToPreparePeriod
Namespace Prefix:	mlp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Gain (Loss) From Equity Interest Transfer

+ References

No definition available.

+ Details

Name:	mlp_GainLossFromEquityInterestTransfer
Namespace Prefix:	mlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Maximum Exposure, Undiscounted

+ References

No definition available.

+ Details

Name:	mlp_MaximumExposureUndiscounted
Namespace Prefix:	mlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Payments To Fund Cash Calls

+ References

No definition available.

+ Details

Name:	mlp_PaymentsToFundCashCalls
Namespace Prefix:	mlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage Of Ownership

+ References

No definition available.

+ Details

Name:	mlp_PercentageOfOwnership
Namespace Prefix:	mlp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Variable Interest Entity, Reporting Entity Involvement, Maximum Equity Contribution, Amount, Co-venture

+ References

No definition available.

+ Details

Name:	mlp_VariableInterestEntityReportingEntityInvolvementMaximumEquityContributionAmountCoventure
Namespace Prefix:	mlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of the required periodic payments applied to interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentPeriodicPaymentInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ 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 (c)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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=121555522&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=mlp_GrayOakPipelineLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=mlp_DakotaAccessLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=mlp_SeniorNotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=mlp_SeniorNotesDue2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=mlp_SeniorNotesDue2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=mlp_CommonControlTransactionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_MajorityShareholderMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=mlp_Phillips66PDIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis=mlp_LibertyPipelineLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mlp_LibertyPipelineLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mlp_GrayOakPipelineLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=mlp_ThirdPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mlp_GrayOakHoldingsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mlp_DakotaAccessandETCOMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Equity Investments (Narrative) (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
Equity Investments (Narrative) (Details) - USD ($)	Jul. 06, 2020	Feb. 29, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jan. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Feb. 28, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Gain from equity interest transfer			$ 84,000,000	$ 0	$ 84,000,000	$ 0
Equity investments			$ 3,340,000,000		$ 3,340,000,000			$ 2,961,000,000
Gray Oak Pipeline, LLC
Schedule of Equity Method Investments [Line Items]
Maximum equity contribution, co-venture							$ 1,379,000,000
Dakota Access, LLC \| Subsequent Event
Schedule of Equity Method Investments [Line Items]
Estimated period of time to prepare Environmental Impact Statement	13 months
Dakota Access, LLC \| Senior Notes
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount									$ 2,500,000,000
Dakota Access, LLC \| Senior Notes \| Subsequent Event
Schedule of Equity Method Investments [Line Items]
Scheduled annual interest payments	$ 25,000,000
Dakota Access, LLC \| Senior Notes \| Senior Notes Due 2022
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount									$ 650,000,000
Interest rate, stated percentage									3.625%
Dakota Access, LLC \| Senior Notes \| Senior Notes Due 2024
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount									$ 1,000,000,000.0
Interest rate, stated percentage									3.90%
Dakota Access, LLC \| Senior Notes \| Senior Notes Due 2029
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount									$ 850,000,000
Interest rate, stated percentage									4.625%
Common Control Transaction \| Phillips 66 \| Phillips 66 PDI \| Liberty Pipeline LLC (Liberty)
Schedule of Equity Method Investments [Line Items]
Controlling interest acquired, percentage		50.00%
Business combination, consideration transferred		$ 75,000,000
Liberty Pipeline LLC (Liberty)
Schedule of Equity Method Investments [Line Items]
Ownership interest acquired, percentage			50.00%		50.00%
Equity investments			$ 202,000,000		$ 202,000,000			0
Guarantor obligations, current carrying value			$ 49,000,000		$ 49,000,000
Gray Oak Pipeline, LLC
Schedule of Equity Method Investments [Line Items]
Ownership interest acquired, percentage			65.00%		65.00%					65.00%
Effective ownership interest			42.25%		42.25%
Gain from equity interest transfer			$ 84,000,000
Maximum exposure			583,000,000		$ 583,000,000
Equity investments			896,000,000		896,000,000			$ 759,000,000
Gray Oak Pipeline, LLC \| Third Party
Schedule of Equity Method Investments [Line Items]
Amount contributed to fund cash calls					61,000,000
Gray Oak Holdings LLC \| Third Party
Schedule of Equity Method Investments [Line Items]
Percentage of ownership											35.00%
Dakota Access and ETCO
Schedule of Equity Method Investments [Line Items]
Maximum exposure, undiscounted			$ 631,000,000		$ 631,000,000