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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) - PNM	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) MWh MMBTU	Dec. 31, 2016 MWh MMBTU	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Net exposure | $	$ 0		$ 0
Commodity derivatives | Fair value hedging | Buy
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MMBTU	405,000	100,000
Commodity derivatives | Fair value hedging | Sell
Derivative [Line Items]
Economic Hedges (in mmbtu and mwh) | MWh	40,800	0

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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

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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- Definition

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

+ References

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