Regulatory and Rate Matters - PNM (Details)								1 Months Ended	3 Months Ended			6 Months Ended	12 Months Ended
	Apr. 15, 2020 USD ($) MMBTU numberOfAMIMeters	Feb. 01, 2018	Jan. 10, 2018 USD ($)	Jul. 03, 2017	Dec. 07, 2016 USD ($)	Sep. 28, 2016 USD ($) lease MW	Aug. 27, 2015 USD ($)	Aug. 31, 2016 USD ($) lease MW	Mar. 31, 2020 USD ($) MW	Sep. 30, 2019	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) GWh	Dec. 31, 2017	Mar. 27, 2020 MW	May 16, 2019 MW	Mar. 31, 2017 MW	Jan. 31, 2016 MW
Public Utilities, Commitments And Contingencies [Line Items]
Income taxes (benefit)									$ 1,880,000		$ (1,223,000)
Regulatory disallowances and restructuring costs									$ 0		1,345,000
Percent of non-fuel revenue requirement change implemented		50.00%
Excess return on jurisdictional equity that would require refund									0.50%
PNM
Public Utilities, Commitments And Contingencies [Line Items]
Income taxes (benefit)									$ 2,359,000		(1,973,000)
Regulatory disallowances and restructuring costs									$ 0		1,345,000
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
Current output in the geothermal facility (in megawatts) | MW									15
Solar generation capacity (in megawatts) | MW									133.0
Renewable Energy Rider | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Recorded revenues from renewable rider									$ 15,100,000		12,700,000
Revenue from renewable energy rider, year two									$ 58,900,000
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
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%
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%
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%
Minimum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Profit incentive sliding scale multiplier													0.071
Maximum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Reasonable cost threshold in megawatts per hour									$ 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
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
Portion of purchase price									$ 73,200,000
Income taxes (benefit)									45,700,000
Leaseholds and Leasehold Improvements | New Mexico 2015 Rate Case | Palo Verde Nuclear Generating Station, Unit 1 and 4 Leases | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Undepreciated capitalized improvements									$ 37,700,000
Subsequent event | 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 | numberOfAMIMeters	5,000
Program costs related to meter installation	$ 2,900,000
Subsequent event | Minimum | Renewable Portfolio Standard | PNM
Public Utilities, Commitments And Contingencies [Line Items]
Profit incentive sliding scale multiplier	0.071
Energy efficiency and load management programs targeted energy savings (in Gigawatts) | MMBTU	80
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