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Fair Value of Derivative and Other Financial Instruments - Sale of Power (Details) - PNMR and PNM [Member] - Palo Verde Nuclear Generating Station Unit 3 [Member]	Dec. 31, 2017 $ / MWh	Dec. 31, 2016 MW
Sale of Power [Line Items]
Number of megawatts not included in retail rates (in megawatts) | MW		134
Percentage of electric power plant output sold or contracted to sell		100.00%
Commodity Contract [Member] | Subsequent Event [Member]
Sale of Power [Line Items]
Derivative, average forward price (in dollars per megawatt hour) | $ / MWh	29

X

- Definition

Percentage of Electric Power Plant Output Sold through 2017

+ References

No definition available.

+ Details

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Namespace Prefix:	pnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Public Utilities, Number of Megawatts Nuclear Generation

+ References

No definition available.

+ Details

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Namespace Prefix:	pnm_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Sale of Power [Line Items]

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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

X

- Details

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

X

- Details

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