Regulatory and Rate Matters - PNM (Details)											1 Months Ended	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Oct. 14, 2020	Jun. 01, 2020 USD ($)	May 28, 2020 USD ($)	Apr. 15, 2020 USD ($) MMBTU meter	Feb. 01, 2018	Jan. 10, 2018 USD ($)	Jul. 03, 2017	Dec. 07, 2016 USD ($)	Sep. 28, 2016 USD ($) lease MW	Aug. 27, 2015 USD ($)	Aug. 31, 2016 USD ($) lease MW	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) GWh	Dec. 31, 2017	Mar. 27, 2020 MW	May 16, 2019 MW	Sep. 30, 2016 MW	Jan. 31, 2016 MW
Public Utilities, Commitments And Contingencies [Line Items]
Income taxes (benefit)												$ (12,331,000)	$ (9,188,000)			$ (14,726,000)	$ 32,420,000
Regulatory disallowances and restructuring costs												$ 0	0			$ 0	150,599,000
Percent of non-fuel revenue requirement change implemented					50.00%
Excess return on jurisdictional equity that would require refund												0.50%				0.50%
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Income taxes (benefit)												$ (13,591,000)	(9,525,000)			$ (16,127,000)	31,984,000
Regulatory disallowances and restructuring costs												$ 0	$ 0			$ 0	150,599,000
Solar generation capacity (in megawatts) | MW																				300
Action plan, covered period							4 years
2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)										$ 123,500,000
Requested rate increase (decrease) of non-fuel revenue										$ 121,700,000
Approved rate increase (decrease)									$ 61,200,000
Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts of Solar PV facilities | MW												157				157
Current output in the geothermal facility (in megawatts) | MW												15				15
Solar generation capacity (in megawatts) | MW												145.0				145.0
Energy Efficiency and Load Management Program | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Energy efficiency and load management programs targeted energy savings (in Gigawatts) | MMBTU				80
Program costs related to energy efficiency, in year two				$ 31,400,000
Program costs related to energy efficiency, in year three				31,000,000.0
Program costs related to energy efficiency, in year four				$ 29,600,000
Number of AMI meters to be installed | meter				5,000
Program costs related to meter installation				$ 2,900,000
Integrated Resource Plan, 2011 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Frequency of IRP filings																3 years
Planning period covered of IRP																20 years
Integrated Resource Plan, 2017 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Planning period covered of IRP							20 years
Integrated Resource Plan, 2020 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Planning period covered of IRP													20 years
NMPRC
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)								$ 99,200,000
Requested return on equity								10.125%
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate																			3 years
Action plan, covered period																4 years
NMPRC | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)						$ 10,300,000
Requested return on equity						9.575%
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate						3 years
Proposed revision to rider that will allow for recovery																		$ 49,600,000
NMPRC | Renewable Energy Rider | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Proposed revision to rider that will allow for recovery		$ 67,800,000
Revenue from renewable energy rider, year two																$ 58,900,000
Recorded revenues from renewable rider												$ 12,000,000.0	$ 11,900,000			$ 42,300,000	$ 37,400,000
NMPRC | Renewable Energy Rider, Sky Blue Energy Program | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Proposed revision to rider that will allow for recovery		$ 2,300,000
Renewable energy procurement plan, carrying charges, percent		8.64%
Renewable energy procurement plant, carrying charges		$ 700,000
NMPRC | Renewable Energy Rider, Sky Blue Energy Program | PNM | Subsequent event
Public Utilities, Commitments And Contingencies [Line Items]
Renewable energy procurement plan, carrying charges, percent	4.00%
Required Percentage by 2011 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales												10.00%				10.00%
Required Percentage by 2015 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales												15.00%				15.00%
Required Percentage by 2020 | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Required percentage of renewable energy in portfolio to electric sales												20.00%				20.00%
Minimum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Profit incentive sliding scale multiplier				0.071														0.071
Energy efficiency and load management programs targeted energy savings (in Gigawatts) | MMBTU				80
Maximum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Reasonable cost threshold in megawatts per hour												$ 60				$ 60
Palo Verde Nuclear Generating Station, Unit 2 Leases | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts purchased (in megawatts) | MW									64.1		64.1											64.1	64.1
Number of megawatts nuclear generation (in megawatts) | MW									114.6		114.6										114.6	114.6
Number of leases under which assets were purchased | lease									3		3
Public utilities, aggregate annual rent expense											$ 18,100,000
Palo Verde Nuclear Generating Station, Unit 2 Leases | 2015 Electric Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Hearing examiner's proposed disallowance of recovery											$ 163,300,000
Initial rate base value									$ 83,700,000
Disallowance of the recovery undepreciated costs of capitalized leasehold improvements									$ 43,800,000
Pre-tax regulatory disallowance														$ 19,700,000
New Mexico Wind | Renewable Portfolio Standard 2014 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts for wind energy | MW																204
Red Mesa Wind | Renewable Portfolio Standard 2014 | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts for wind energy | MW																102
La Joya Wind | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts for wind energy | MW																140
Four Corners
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory disallowances and restructuring costs						$ 47,600,000
Regulatory disallowance						$ 148,100,000
New Mexico 2015 Rate Case | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Pre-tax impairment of investments														$ 1,300,000	$ 150,600,000
Undepreciated capitalized improvements												$ 39,700,000				$ 39,700,000
New Mexico 2015 Rate Case | Palo Verde Nuclear Generating Station, Unit 2 Leases | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Number of megawatts purchased (in megawatts) | MW												64.1				64.1					64.1
Portion of purchase price																$ 73,200,000
Income taxes (benefit)																45,700,000
New Mexico 2015 Rate Case | Palo Verde Nuclear Generating Station, Unit 1 and 4 Leases | PNM | Leaseholds and Leasehold Improvements
Public Utilities, Commitments And Contingencies [Line Items]
Undepreciated capitalized improvements												$ 37,700,000				$ 37,700,000
Disincentives and Incentives Added | 2017 Energy Efficiency and Load Management Program | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Energy efficiency and load management programs profit incentive, minimum																		$ 1,800,000
Energy efficiency and load management programs targeted energy savings (in Gigawatts) | GWh																		59
Energy efficiency and load management programs additional incentive targeted energy savings (in Gigawatts) | GWh																		68
Disincentives and Incentives Added | 2019 Energy Efficiency and Load Management Program | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Energy efficiency and load management programs profit incentive, minimum			$ 500,000