Regulatory and Rate Matters - SJGS, Fours Corners and PVNGS Leased Interest Abandonment Applications (Details) $ in Thousands						1 Months Ended	3 Months Ended	9 Months Ended
	Apr. 02, 2021 lease MW	Feb. 03, 2021	Jan. 08, 2021 USD ($)	Nov. 01, 2020 USD ($)	Jul. 01, 2019 USD ($)	Nov. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MW power_purchase_agreement	Sep. 30, 2021 USD ($) MW	Oct. 01, 2021 MW	Jul. 28, 2021 MW	Jul. 22, 2021 resource_project MW	Jun. 23, 2021 MW	May 27, 2021 MW	Mar. 12, 2021 MW	Feb. 08, 2021 MW	Jan. 28, 2021 MW	Dec. 31, 2020 USD ($)	Dec. 18, 2020 USD ($)	Nov. 13, 2020 MW	Sep. 28, 2020 USD ($)	Jul. 29, 2020 MW	Mar. 31, 2020 USD ($)	Feb. 21, 2020 USD ($)
Public Utilities, General Disclosures [Line Items]
Number of purchase power agreements | power_purchase_agreement							3
Regulatory time frame for proposed PPAs and battery storage agreements	6 months
Ownership							50.00%	50.00%
Regulatory assets							$ 544,432	$ 544,432									$ 557,790
Regulatory liabilities							$ 855,855	855,855									850,228
COVID-19
Public Utilities, General Disclosures [Line Items]
Temporary mandatory moratorium on disconnection period		100 days
Disconnection transition period		90 days
Facebook Data Center
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW										190					190
Number of megawatts in battery storage facilities (in megawatts) | MW										100		100			100
Solar generation capacity expected to be operational (in megawatts) | MW										50					50
Number of megawatts in battery storage facilities approved (in megawatts) | MW										50
Facebook Data Center | Subsequent event
Public Utilities, General Disclosures [Line Items]
Number of megawatts in battery storage facilities (in megawatts) | MW									100
Number of megawatts in battery storage facilities approved (in megawatts) | MW									50
PNM
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW																					650
Number of megawatts in battery storage facilities (in megawatts) | MW																					300
Number of additional approved replacement resource projects | resource_project											2
Number of purchase power agreements | power_purchase_agreement							3
Number of leases under which lease term was extended | lease	5
Regulatory assets							$ 452,596	452,596									457,953
Regulatory liabilities							661,272	661,272									664,873
Solar generation capacity related to unexecuted transmission service agreement (in megawatts) | MW														145
PNM | Four Corners
Public Utilities, General Disclosures [Line Items]
Request issuance of energy transition bonds			$ 300,000
Forecasted undepreciated investment			272,000
Plant decommissioning and coal mine reclamation costs			4,600
Upfront financing costs			7,300
Proceeds from securitization bonds			$ 16,500
Solar generation capacity (in megawatts) | MW																200
Payments for relief from obligations				$ 75,000		$ 15,000
Final payment for relief from obligations						60,000
Initial payment for relief from obligations						$ 15,000
PNM | Four Corners | PNM
Public Utilities, General Disclosures [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners				13.00%
PNM | COVID-19 | COVID-19 Costs Savings
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities																	900
PNM | Deferred COVID-19 Costs | COVID-19
Public Utilities, General Disclosures [Line Items]
Regulatory assets							$ 13,200	$ 13,200									8,800
PNM | NMPRC
Public Utilities, General Disclosures [Line Items]
Solar generation capacity, amount requested for approval to abandon (in megawatts) | MW	114
PNM | Leased Interest Termination on January 15, 2023
Public Utilities, General Disclosures [Line Items]
Number of leases under which lease term was extended | lease	4
PNM | Leased Interest Termination on January 15, 2023 | NMPRC
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW	104
PNM | Leased Interest Termination on January 15, 2024 | NMPRC
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW	10
PNM | Replacement Resource Portfolio One
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW	150										200								200
Number of megawatts in battery storage facilities (in megawatts) | MW	40										100								100
Number of megawatts available in battery storage facilities (in megawatts) | MW											50
PNM | Replacement Resource Portfolio Two
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW											300		100						100
Number of megawatts in battery storage facilities (in megawatts) | MW	100										150		30						30
Number of megawatts available in battery storage facilities (in megawatts) | MW											150
Number of megawatts available in solar facilities (in megawatts) | MW											150
PNM | Replacement Resource Portfolio Three
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW	300
Number of megawatts in battery storage facilities (in megawatts) | MW	150
PNM | Transportation Electrification Program
Public Utilities, General Disclosures [Line Items]
Expected budgeted expenditures																		$ 8,400
Budget flexibility																		25.00%
Budget portion dedicated to low and moderate income customers																		25.00%
PNM | Portfolio One
Public Utilities, General Disclosures [Line Items]
Number of megawatts available in purchase power agreement | MW							85	85
PNM | Portfolio Two
Public Utilities, General Disclosures [Line Items]
Number of megawatts available in purchase power agreement | MW							150	150
PNM | Portfolio Three
Public Utilities, General Disclosures [Line Items]
Number of megawatts available in purchase power agreement | MW							40	40
PNMR Development
Public Utilities, General Disclosures [Line Items]
Solar generation capacity (in megawatts) | MW							130	130
Ownership							50.00%	50.00%
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]
Request issuance of energy transition bonds					$ 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