Regulatory and Rate Matters - SJGS Abandonment Application (Details) $ in Thousands						9 Months Ended
	Oct. 02, 2020 USD ($)	Apr. 01, 2020 USD ($)	Jul. 10, 2019	Jul. 01, 2019 USD ($) scenario MW	May 31, 2019 MW	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)	Sep. 28, 2020 USD ($) MW	Jun. 24, 2020 MW	Mar. 31, 2020 USD ($)	Mar. 27, 2020 MMBTU MW	Feb. 21, 2020 USD ($)	Aug. 10, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Increase (decrease) in allowance for equity funds used during construction | $						$ 6,728	$ 6,714
NMPRC
Public Utilities, General Disclosures [Line Items]
Deadline extension period to issue abandonment and financing order			9 months
Deadline extension period to issue replacement resources order			15 months
COVID-19
Public Utilities, General Disclosures [Line Items]
Deferred incremental costs | $						$ 3,600
PNM Solar Direct
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts)					50
Power purchase agreement term					15 years
Replacement Resource Portfolio Two | Hearing Examiners Approach Two
Public Utilities, General Disclosures [Line Items]
Number of megawatts for natural gas facility (in Megawatts)									200
Replacement Resource Portfolio Four | Hearing Examiners Approach One
Public Utilities, General Disclosures [Line Items]
Number of megawatts for demand response (in Megawatts)									24
KV Transmission Line
Public Utilities, General Disclosures [Line Items]
Estimated cost of project | $													$ 85,000
KV Transmission Line | Subsequent event
Public Utilities, General Disclosures [Line Items]
Estimated cost of project | $	$ 109,000
Increase (decrease) in estimated cost of project | $	$ 24,000
Required Percentage by 2020 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Renewable energy, percentage						0.20
Required Percentage by 2025 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Renewable energy, percentage						0.40
Required Percentage by 2030 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Renewable energy, percentage						0.50
Required Percentage by 2040 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Renewable energy, percentage						0.80
Required Percentage by 2045 | Energy Transition Act | Electric-Generation Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Renewable energy, percentage						1
PNM
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)											40
Solar generation capacity (in megawatts)											300
Solar generation capacity, power purchase agreements (in Megawatts) | MMBTU											20
Increase (decrease) in allowance for equity funds used during construction | $						$ 4,229	$ 4,904
PNM | Audit Of PNM’s Compliance With OATT And Financial Reporting
Public Utilities, General Disclosures [Line Items]
Interest expense, debt | $						1,900
Increase (decrease) in allowance for equity funds used during construction | $						$ 100
PNM | Renewable Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts)						145.0
PNM | PNM Solar Direct
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts)											50
PNM | Replacement Resource Portfolio One | Hearing Examiners Approach One
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)									150
Solar generation capacity (in megawatts)									300
PNM | Replacement Resource Portfolio Two | Hearing Examiners Approach One
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)									20
Solar generation capacity (in megawatts)									50
PNM | Replacement Resource Portfolio Three
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)								100
Solar generation capacity (in megawatts)								200
PNM | Replacement Resource Portfolio Three | Hearing Examiners Approach One
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)									100
Solar generation capacity (in megawatts)									200
PNM | Replacement Resource Portfolio Four
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)								30
Solar generation capacity (in megawatts)								100
PNM | Replacement Resource Portfolio Four | Hearing Examiners Approach One
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)									30
Solar generation capacity (in megawatts)									100
PNM | Replacement Resource Portfolio Five
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts)								130
Solar generation capacity (in megawatts)								299
PNM | Required Percentage by 2011 | Renewable Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Required percentage of renewable energy in portfolio to electric sales						10.00%
PNM | Required Percentage by 2015 | Renewable Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Required percentage of renewable energy in portfolio to electric sales						15.00%
PNM | Required Percentage by 2020 | Renewable Portfolio Standard
Public Utilities, General Disclosures [Line Items]
Required percentage of renewable energy in portfolio to electric sales						20.00%
San Juan Generating Station
Public Utilities, General Disclosures [Line Items]
Estimated costs of replacement resources | $								$ 8,100
San Juan Generating Station | Other deferred credits
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $						$ 38,100
San Juan Generating Station | Employee Severance
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $										$ 9,400
San Juan Generating Station | PNM
Public Utilities, General Disclosures [Line Items]
Number of megawatts in natural gas-fired peaking plant (in megawatts)				280
Number of megawatts in battery storage facilities (in megawatts)				70
Solar generation capacity (in megawatts)				350
Number of megawatts in battery storage facilities, replacement resource scenario (in megawatts)				60
Number of replacement resource scenarios | scenario				3
Request issuance of energy transition bonds | $		$ 361,000		$ 361,000
Forecasted undepreciated investment | $				283,000								$ 361,000
Plant decommissioning and coal mine reclamation costs | $				28,600
Upfront financing costs | $				9,600
Severance costs | $				20,000
Proceeds from securitization bonds | $				$ 19,800
San Juan Generating Station | PNM | Other deferred credits
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $						$ 36,800
San Juan Generating Station | PNM | Employee Severance
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $										8,100
San Juan Generating Station | PNM | Other Restructuring
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $										8,900
San Juan Generating Station | PNM | Economic Development and Workforce Training Costs
Public Utilities, General Disclosures [Line Items]
Restructuring reserve | $										$ 19,800