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Regulatory and Rate Matters - Renewable Energy Rider (Details) - Public Service Company of New Mexico - USD ($)
$ in Millions
 12 Months Ended
Jan. 01, 2021
Jun. 01, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Renewable Energy Rider          
Public Utilities, General Disclosures [Line Items]          
Revenue from renewable energy rider     $ 56.4 $ 52.0 $ 41.4
Renewable Energy Rider | NMPRC          
Public Utilities, General Disclosures [Line Items]          
Proposed revision to rider that will allow for recovery   $ 67.8      
Renewable Energy Rider | NMPRC | Subsequent Event          
Public Utilities, General Disclosures [Line Items]          
Proposed revision to rider that will allow for recovery $ 67.8        
Renewable energy procurement plan, approved recovery amount 65.5        
Renewable Energy Rider | Maximum          
Public Utilities, General Disclosures [Line Items]          
NMPRC-approved return on equity     0.50%    
Renewable Energy Rider, Sky Blue Energy Program | NMPRC          
Public Utilities, General Disclosures [Line Items]          
Proposed revision to rider that will allow for recovery   $ 2.3      
Renewable Energy Rider, Sky Blue Energy Program | NMPRC | Subsequent Event          
Public Utilities, General Disclosures [Line Items]          
Proposed revision to rider that will allow for recovery $ 2.3