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Fair Value of Derivative and Other Financial Instruments - Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 587,530	$ 531,467	$ 489,978
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	93,402	86,400	69,703
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	498,907	445,984
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	587,891	532,659
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	88,984	86,675
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	93,710	86,749
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,310	1,022
Public Service Company of New Mexico | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	92,400	85,727
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	241,445	152,158
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	53,594	40,383
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,310	1,022
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	52,284	39,361
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	257,462	293,826
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	40,116	46,366
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	40,116	46,366
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	66,149	59,367	55,074
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	12,885	10,844	$ 8,744
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	56,966	49,353
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	66,196	59,327
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	9,230	9,974
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	12,861	10,910
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	18	275
Texas-New Mexico Power Company | Fair Value, Inputs, Level 1, 2 and 3 | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	12,843	10,635
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	28,732	17,335
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	10,824	4,350
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	18	275
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	10,806	4,075
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	28,234	32,018
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	2,037	6,560
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	2,037	6,560
PNMR
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 654,087	$ 591,986
PNMR | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
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PNMR | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
PNMR | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Fair Value by Hierarchy and NAV [Extensible List]	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember	us-gaap:FairValueMeasuredAtNetAssetValuePerShareMember
PNMR | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 555,873	$ 495,337
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	20,812	19,982
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | International funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	114,983	68,497
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Domestic value
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,187	825
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Domestic growth
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	173,931	172,326
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | U.S. government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	97,460	90,970
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | International government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	6,202	5,411
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	6,277	6,980
PNMR | Fair Value, Inputs, Level 1, 2 and 3 | Corporate and other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	135,021	130,346
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	270,177	169,493
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	20,812	19,982
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | International funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	114,983	68,497
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic value
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	135	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic growth
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	47,418	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	86,829	81,014
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | International government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipals
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate and other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
PNMR | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	285,696	325,844
PNMR | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
PNMR | Significant Other Observable Inputs (Level 2) | International funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	0	0
PNMR | Significant Other Observable Inputs (Level 2) | Domestic value
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	1,052	825
PNMR | Significant Other Observable Inputs (Level 2) | Domestic growth
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	126,513	172,326
PNMR | Significant Other Observable Inputs (Level 2) | U.S. government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	10,631	9,956
PNMR | Significant Other Observable Inputs (Level 2) | International government
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	6,202	5,411
PNMR | Significant Other Observable Inputs (Level 2) | Municipals
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	6,277	6,980
PNMR | Significant Other Observable Inputs (Level 2) | Corporate and other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	135,021	130,346
PNMR | Fair Value Measured at Net Asset Value Per Share | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	12,552	15,827
PNMR | Fair Value Measured at Net Asset Value Per Share | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	52,285	47,618
PNMR | Fair Value Measured at Net Asset Value Per Share | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets, amount	$ 33,377	$ 33,204

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