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Electric Operating Revenues - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 16, 2015 MW
PNM
Disaggregation of Revenue [Line Items]
Alternative revenue programs	$ 1,100
Other electric operating revenue	28,400
TNMP
Disaggregation of Revenue [Line Items]
Alternative revenue programs	3,500
Other electric operating revenue	$ 0
Customer contracts | PNM
Disaggregation of Revenue [Line Items]
Accounts receivable		$ 58,700	$ 62,900
Clean Air Act, SNCR | Palo Verde Nuclear Generating Station Unit 3 | PNM
Disaggregation of Revenue [Line Items]
Number of megawatts nuclear generation (in megawatts) | MW				134

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