Electric Operating Revenues - Narrative (Details)	Sep. 30, 2018 USD ($) utility MW	Dec. 31, 2017 USD ($) MW
Disaggregation of Revenue [Line Items]
Number of regulated utilities \| utility	2
Contract assets \| $	$ 0
PNM
Disaggregation of Revenue [Line Items]
Expected exposure to market risk (in megawatts)	65
Power to be sold to third party (in megawatts)	36
Customer contracts \| PNM
Disaggregation of Revenue [Line Items]
Accounts receivable \| $	$ 76,900,000	$ 61,800,000
Clean Air Act, SNCR \| Palo Verde Nuclear Generating Station Unit 3 \| PNM
Disaggregation of Revenue [Line Items]
Number of megawatts nuclear generation (in megawatts)		134

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