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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) MMBTU MWh	Dec. 31, 2017 USD ($) MMBTU MWh
Derivative [Line Items]
Contract in a liability position | $	$ 200,000	$ 0
PNM | Commodity derivatives | Fair value hedging | Buy
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MMBTU	330,000	100,000
PNM | Commodity derivatives | Fair value hedging | Sell
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MWh	51,200	0

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- Definition

Contract With Customer, Liability, Net

+ References

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+ Details

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- 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

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- Definition

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

+ References

No definition available.

+ Details

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- Details

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- Details

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