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Regulatory and Rate Matters - Advanced Metering Infrastructure Application and Facebook Data Center Project (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 MW	Dec. 31, 2017 power_purchase_agreement Facility MW	Feb. 08, 2021 MW	Dec. 31, 2020	Mar. 27, 2020 MW	Dec. 31, 2019 MW
Public Utilities, General Disclosures [Line Items]
Ownership percentage				50.00%
Facebook Data Center
Public Utilities, General Disclosures [Line Items]
PPA term	25 years
Facebook Data Center \| Subsequent Event
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)			190
Number of megawatts in battery storage facilities (in megawatts)			100
Solar generation capacity expected to be operational (in megawatts)			50
Public Service Company of New Mexico
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)					300
Number of megawatts in battery storage facilities (in megawatts)					40
PNMR Development
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)						130
Ownership percentage						50.00%
PNMR Development \| Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	50	50
Casa Mesa Wind, LLC \| Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	50	50
Avangrid Renewables, LLC \| Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	166	166
Route 66 Solar Energy Center, LLC \| Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	50	50
NMRD \| Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar distributed generation (in mw)	100	100
Number of additional PPAs \| power_purchase_agreement		2
Number of solar facilities \| Facility		2