Debt Instruments, NMPRC Approved credit facility

No definition available.

Name:	pnm_DebtInstrumentsNMPRCApprovedcreditfacility
Namespace Prefix:	pnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Short Term Debt Due to Related Party

+ References

No definition available.

+ Details

Name:	pnm_ShortTermDebtDueToRelatedParty
Namespace Prefix:	pnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of assets pledged to secure a debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

+ Details

Name:	us-gaap_MaximumLengthOfTimeHedgedInInterestRateCashFlowHedge1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_ShortTermBorrowings
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_ShortTermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

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

- Definition

Amount of unrealized gain (loss) related to the increase or decrease in fair value of interest rate derivatives designated as cash flow hedging instruments, which was recorded in accumulated other comprehensive income to the extent that the cash flow hedge was determined to be effective.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=pnm_SeniorUnsecuredNotesMember
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_ConsolidatedEntitiesAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pnm_PNMR2016OneYearTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=pnm_PNMR2016TwoYearTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=pnm_PNMR2018RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pnm_FirstMortgageBondsDue2019Series2009Aat9point50percentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=pnm_InterestRateContractOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=pnm_InterestRateContractTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=pnm_InterestRateContractThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=pnm_JPMorganChaseBankN.A.andU.S.BankNationalAssociationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pnm_PNM2017TermLoanAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Financing - Short-term Debt and Liquidity (Details)	3 Months Ended	12 Months Ended
Financing - Short-term Debt and Liquidity (Details)	Mar. 31, 2018 USD ($) derivative	Dec. 31, 2017 USD ($)	Nov. 01, 2020 USD ($)	Apr. 23, 2018 USD ($)	Apr. 09, 2018 USD ($)	Mar. 09, 2018 USD ($)	Feb. 26, 2018 USD ($)	Oct. 20, 2017 USD ($)	Jul. 28, 2017 USD ($)	Jul. 20, 2017 USD ($)	Dec. 21, 2016 USD ($)	Nov. 30, 2016 USD ($)
Short-term Debt [Line Items]
Short-term debt	$ 238,700,000	$ 305,400,000
Letters of credit outstanding	6,400,000
Maturities and other repayments due in next 12 months	10,000,000
Maturities and other repayments due in the remainder of year two	9,400,000
Senior Unsecured Notes
Short-term Debt [Line Items]
Debt issued						$ 300,000,000
Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				$ 669,400,000
PNMR
Short-term Debt [Line Items]
Short-term debt – affiliate								$ 0
PNMR \| Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				197,400,000
Consolidated invested cash				900,000
PNMR 2016 One-Year Term Loan (as extended)
Short-term Debt [Line Items]
Short-term debt	$ 100,000,000	100,000,000									$ 100,000,000.0
Weighted-average interest rate for short-term debt	2.63%
Term of loan	1 year
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives	4 years
Debt issued	$ 50,000,000.0
PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 21,500,000	0
PNMR 2016 Two-Year Term Loan
Short-term Debt [Line Items]
Short-term debt											$ 100,000,000.0
Term of loan	2 years
PNMR Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity												$ 10,000,000.0
PNMR Revolving Credit Facility \| PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.05%
PNMR Revolving Credit Facility \| Forecast
Short-term Debt [Line Items]
Financing capacity			$ 290,000,000.0
TNMP Revolving Credit Facility \| PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	2.54%
PNMR Development Revolving Credit Facility \| PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	2.72%
PNM
Short-term Debt [Line Items]
Short-term debt	$ 0	39,800,000
Letters of credit outstanding	2,500,000
Short-term debt – affiliate	54,600,000	0
PNM \| Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				397,500,000
Consolidated invested cash				8,600,000
PNM \| Lines of credit
Short-term Debt [Line Items]
NMPRC approved credit facility	40,000,000
Short-term debt	0	0
PNM \| Lines of credit \| Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				40,000,000
PNM \| Senior Unsecured Note Agreement (SUNs) \| Senior Unsecured Notes
Short-term Debt [Line Items]
Debt issued									$ 450,000,000.0
PNM \| PNM Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity												$ 40,000,000.0
PNM \| PNM Revolving Credit Facility \| Forecast
Short-term Debt [Line Items]
Financing capacity			$ 360,000,000.0
TNMP
Short-term Debt [Line Items]
Short-term debt	21,200,000	0
Letters of credit outstanding	100,000
Short-term debt – affiliate	2,600,000	0
TNMP \| First Mortgage Bonds
Short-term Debt [Line Items]
Debt issued	172,300,000
TNMP \| Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity				34,500,000
Consolidated invested cash				0
PNMR Development \| Subsequent event
Short-term Debt [Line Items]
Short-term debt – affiliate					$ 68,200,000
Remaining borrowing capacity				0
PNMR Development \| PNMR 2018 Revolving Credit Facility
Short-term Debt [Line Items]
Financing capacity							$ 24,500,000.0
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity	300,000,000.0
Short-term debt	96,000,000	165,600,000
Revolving credit facility \| PNM
Short-term Debt [Line Items]
Financing capacity	400,000,000.0
Short-term debt	0	39,800,000
Revolving credit facility \| TNMP
Short-term Debt [Line Items]
Financing capacity	75,000,000.0
Short-term debt	21,200,000	0
Revolving credit facility \| TNMP \| First mortgage bonds
Short-term Debt [Line Items]
Collateral amount	$ 75,000,000
Affiliated entity \| TNMP \| Subsequent event
Short-term Debt [Line Items]
Short-term debt – affiliate				$ 0
Interest rate contract
Short-term Debt [Line Items]
Number of derivatives \| derivative	3
Interest rate 1 \| Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate	1.926%
Interest rate 2 \| Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate	1.823%
Interest rate 3 \| Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate	1.629%
Interest rate 3 \| Level 2 \| Cash Flow Hedge
Short-term Debt [Line Items]
Fair value gain (loss)	$ 3,100,000	$ 1,400,000
JPMorgan Chase Bank, N.A. and U.S. Bank National Association \| PNM \| PNM 2017 Term Loan Agreement \| Term loan agreement with banks
Short-term Debt [Line Items]
Debt issued										$ 200,000,000.0