Commitments and Contingencies - The Clean Air Act (Details)		1 Months Ended	35 Months Ended
	Jul. 17, 2015	Dec. 31, 2015 USD ($) MWh MW	Jan. 31, 2016 USD ($)	Dec. 31, 2019 USD ($) lb / MMBTU joint_owner T MW	Jul. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($) joint_owner MW	Feb. 09, 2016 state	Aug. 06, 2012 compliance_alternative	Dec. 31, 1999 state
Public Utilities, General Disclosures [Line Items]
Plant in service, held for future use, and to be abandoned				$ 7,918,601,000		$ 7,548,581,000
Accumulated depreciation and amortization				2,713,503,000		2,604,177,000
Regulatory assets				$ 556,930,000		598,930,000
San Juan Generating Station
Public Utilities, General Disclosures [Line Items]
Number of other entities (in joint owner) | joint_owner				5
Four Corners
Public Utilities, General Disclosures [Line Items]
Approved lease extension term	25 years
Public Service Company of New Mexico
Public Utilities, General Disclosures [Line Items]
Plant in service, held for future use, and to be abandoned				$ 5,753,267,000		5,623,520,000
Accumulated depreciation and amortization				2,076,291,000		2,006,266,000
Regulatory assets				$ 435,467,000		$ 460,903,000
Power to be sold to third party (in megawatts) | MW				36			36
Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3
Public Utilities, General Disclosures [Line Items]
Recovery percentage of estimated undepreciated value							50.00%
Plant in service, held for future use, and to be abandoned							$ 439,400,000
Accumulated depreciation and amortization							188,300,000
Net book value							251,100,000
Regulatory assets							125,500,000
Reversal of plant write-off							3,000,000.0
Reversal of recorded loss for other unrecoverable costs							1,000,000.0
Total reversed losses							$ 4,000,000.0
Public Service Company of New Mexico | San Juan Generating Station
Public Utilities, General Disclosures [Line Items]
Forecasted undepreciated investment					$ 283,000,000
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment
Public Utilities, General Disclosures [Line Items]
Estimate of possible loss				$ 92,600,000
Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal)
Public Utilities, General Disclosures [Line Items]
Plant in service, held for future use, and to be abandoned				283,939,000
Accumulated depreciation and amortization				$ 100,137,000
Clean Air Act related to Regional Haze
Public Utilities, General Disclosures [Line Items]
Number of states to address regional haze | state										50
Potential to emit tons per year of visibility impairing pollution, maximum (in tons) | T				250
Clean Air Act, SNCR | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Number of other entities (in joint owner) | joint_owner							8
Clean Air Act, SNCR | San Juan Generating Station Unit 4 | CALIFORNIA
Public Utilities, General Disclosures [Line Items]
Number of other entities (in joint owner) | joint_owner							3
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Units 2 and 3
Public Utilities, General Disclosures [Line Items]
Current ownership (in megawatts) | MW		418
Requested time period to recover retired units NBV		20 years
Recovery percentage of estimated undepreciated value		50.00%
Accumulated plant write-off, disallowance							$ 128,600,000
Clean Air Act, SNCR | Public Service Company of New Mexico | Palo Verde Nuclear Generating Station Unit 3
Public Utilities, General Disclosures [Line Items]
Number of megawatts (in megawatts) | MW							134
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station
Public Utilities, General Disclosures [Line Items]
Additional ownership to be obtained (in megawatts) | MW		132				132
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Additional ownership to be obtained (in megawatts) | MW		132
Estimated rate base value		$ 0
Coal-fired generation (in megawatts) | MW		197
Number of megawatt hours of renewable energy certificates to be acquired and retired (in megawatt hours) | MWh		1
Potential acquisition of ownership (in megawatts) | MW		65				65
Pre-tax impairment of investments						$ 35,000,000.0
Undepreciated investment in ownership to be obtained						11,900,000
Forecasted undepreciated investment						$ 23,100,000
Percentage of ownership held by exiting owners							38.80%
Ownership percentage							38.50%
Clean Air Act, SNCR | Public Service Company of New Mexico | San Juan Generating Station Unit 3
Public Utilities, General Disclosures [Line Items]
Percentage of ownership held by exiting owners							50.00%
Ownership percentage							50.00%
Clean Air Act, SNCR | Public Service Company of New Mexico | Maximum | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Annual cost of renewable energy credits, maximum		$ 7,000,000.0
Clean Air Act, SNCR | Public Service Company of New Mexico | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs | San Juan Generating Station Units 1 and 4
Public Utilities, General Disclosures [Line Items]
Portion of costs for SNCRs and BDT equipment			$ 77,700,000
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Units 1 and 4
Public Utilities, General Disclosures [Line Items]
Ownership percentage				66.30%
Clean Air Act, SNCR | Public Service Company of New Mexico | Scenario, plan | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Ownership percentage				77.30%
Clean Air Act, SNCR | PNMR and PNM | Palo Verde Nuclear Generating Station Unit 3
Public Utilities, General Disclosures [Line Items]
Number of megawatts (in megawatts) | MW		134
Clean Air Act, SNCR | PNMR Development | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Additional ownership to be obtained (in megawatts) | MW							132
Potential acquisition of ownership (in megawatts) | MW		65				65	65
Clean Air Act, SNCR Hearing Examiner Recommended Denial | Public Service Company of New Mexico | San Juan Generating Station Unit 4
Public Utilities, General Disclosures [Line Items]
Additional ownership to be obtained (in megawatts) | MW		132				132
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners
Public Utilities, General Disclosures [Line Items]
Number of compliance alternatives | compliance_alternative									2
Government standard emissions limit (in pounds per MMBTU) | lb / MMBTU				0.015
Plant requirement to meet opacity limit				20.00%
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations				20.00%
Estimate of possible loss						$ 88,700,000
Clean Air Act Related to Post Combustion Controls | Public Service Company of New Mexico | Four Corners Units 4 and 5 (Coal)
Public Utilities, General Disclosures [Line Items]
Ownership percentage				13.00%
Clean Power Plan
Public Utilities, General Disclosures [Line Items]
Number of states that filed a petition against the Clean Power Plan | state								29