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Fair Value of Derivative and Other Financial Instruments - Volume Positions and Requirements to Provide Collateral (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) MMBTU MWh	Dec. 31, 2017 USD ($) MMBTU MWh
Derivative [Line Items]
Contract in a net liability position | $	$ 0	$ 0
Long | Commodity Contract | Economic Hedges | PNMR and PNM
Derivative [Line Items]
Power-related contracts | MMBTU	100,000	100,000
Short | Commodity Contract | Economic Hedges | PNMR and PNM
Derivative [Line Items]
Power-related contracts | MWh	0	0

X

- Definition

Contract With Customer, Liability, Net

+ References

No definition available.

+ Details

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Namespace Prefix:	pnm_
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

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

X

- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: