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Commitments and Contingencies - Liability and Insurance Matters (Details) - Public Service Company of New Mexico - Palo Verde Nuclear Generating Station
 12 Months Ended
Dec. 31, 2018
USD ($)
generating_unit
Public Utilities, General Disclosures [Line Items]  
Number of units (in generating units) | generating_unit 3
Nuclear Plant  
Public Utilities, General Disclosures [Line Items]  
Ownership percentage in nuclear reactor 10.20%
Number of units (in generating units) | generating_unit 3
Maximum potential assessment per incident $ 41,600,000
Annual payment limitation related to incident 6,200,000
Aggregate amount of all risk insurance 2,750,000,000.00
Sublimit amount for non-nuclear property damage losses 2,250,000,000.00
Retrospective premium assessment 5,400,000
Nuclear Plant | Commercial Providers  
Public Utilities, General Disclosures [Line Items]  
Liability insurance coverage 450,000,000
Nuclear Plant | Industry Wide Retrospective Assessment Program  
Public Utilities, General Disclosures [Line Items]  
Liability insurance coverage 13,600,000,000.0
Nuclear Plant | Maximum  
Public Utilities, General Disclosures [Line Items]  
Liability insurance coverage $ 14,100,000,000