Value of input used to measure goodwill.

No definition available.

Name:	amgp_GoodwillMeasurementInput
Namespace Prefix:	amgp_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of input used to measure property and equipment.

+ References

No definition available.

+ Details

Name:	amgp_PropertyAndEquipmentMeasurementInput
Namespace Prefix:	amgp_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the threshold number of barrels of fresh water to be purchased from the entity to trigger a contingent consideration payment.

+ References

No definition available.

+ Details

Name:	amgp_ThresholdNumberOfBarrelsOfWaterToTriggerContingentConsiderationPayment
Namespace Prefix:	amgp_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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

- Definition

Value of input used to measure contingent consideration liability from business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiabilityMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
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_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=amgp_SeniorNotesDue2027Bearing5.75RateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=amgp_SeniorNotesDue2028Bearing5.75RateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=amgp_ValuationTechniqueThirdPartyMarketParticipantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=amgp_ValuationTechniqueDiscountedCashFlowAndComparableCompanyAndMarketValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=amgp_ContingentConsiderationPeriodOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=amgp_ContributionConveyanceAndAssumptionAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=amgp_ContingentConsiderationPeriodTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Measurement (Details) $ in Millions	1 Months Ended
Fair Value Measurement (Details) $ in Millions	Sep. 30, 2015 USD ($) bbl	Mar. 31, 2020 USD ($)	Jan. 31, 2020 USD ($)	Mar. 12, 2019
Level 2 \| 5.375% Senior Notes due 2024
Fair value measurement
Debt instrument fair value		$ 458
Level 2 \| 5.75% Senior Notes Due 2027
Fair value measurement
Debt instrument fair value		416
Level 2 \| 5.75% Senior Notes Due 2028
Fair value measurement
Debt instrument fair value		$ 416
Recurring \| Discount rate \| Third-party market participants
Fair value measurement
Property and Equipment discount rate		0.190
Weighted Average \| Recurring \| Discounted cash flow \| Level 3
Fair value measurement
Business comnination cost of capital				0.141
Weighted Average \| Recurring \| Discounted cash flow, comparable company and market value \| Level 3
Fair value measurement
Goodwill cost of capital		0.180
Contingent Consideration Period One
Fair value measurement
Contingent consideration	$ 125		$ 125
Threshold number of barrels of water to trigger contingent consideration payment \| bbl	176,295,000
Contribution Agreement \| Contingent Consideration Period Two
Fair value measurement
Contingent consideration	$ 125
Threshold number of barrels of water to trigger contingent consideration payment \| bbl	219,200,000