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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 4.00%    
Amortization of gains and losses that are outside the corridor 5 years    
Maximum annual contributions per employee 75.00%    
Employer matching contribution, percent of employees' gross pay 6.00%    
Minimum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Non-matching contribution of eligible compensation based on eligible employee's age 3.00%    
Maximum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Non-matching contribution of eligible compensation based on eligible employee's age 10.00%    
Pension Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected long-term return resulting from effect of 1% change 1.00%    
Expected long-term return resulting from effect of one-percentage point increase (as a percent) 1.00%    
Expected employer contributions to pension plans $ 0    
Pension Plan | Minimum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Weighted average discount rate related to anticipated contributions 2.90%    
Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 35.00%    
Pension Plan | Fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 50.00%    
Pension Plan | Alternative Investments      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 15.00%    
Pension Plan | Public Service Company of New Mexico      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 5.90% 6.86% 6.54%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 5,200,000    
Rate of return for retirement plan 19.30%    
Expected employer contributions in year two $ 10,800,000    
Expected employer contributions in year 3 11,500,000    
Expected employer contributions in year 4 10,600,000    
Actuarial gains (losses) recorded as regulatory assets 21,393,000    
Employer contributions $ 0 $ 0  
Pension Plan | Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 5.90% 6.90% 6.57%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 600,000    
Rate of return for retirement plan 20.40%    
Expected employer contributions in year two $ 0    
Expected employer contributions in year 3 0    
Expected employer contributions in year 4 0    
Actuarial gains (losses) recorded as regulatory assets (3,860,000)    
Employer contributions $ 0 $ 0  
Other Postretirement Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected long-term return resulting from effect of 1% change 1.00%    
Other Postretirement Benefits | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 30.00%    
Other Postretirement Benefits | Fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 70.00%    
Other Postretirement Benefits | Public Service Company of New Mexico      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 7.00% 7.20% 7.42%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 900,000    
Rate of return for retirement plan 11.10%    
Actuarial gains (losses) recorded as regulatory assets $ 600,000    
Employer contributions 3,256,000 $ 3,580,000  
Expected employer disbursements for next fiscal year 3,700,000    
Expected employer disbursements by employer in year 2 through year 5 $ 13,100,000    
Other Postretirement Benefits | Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected return on plan assets 5.60% 5.80% 5.86%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 100,000    
Rate of return for retirement plan 23.60%    
Actuarial gains (losses) recorded as regulatory assets $ 1,200,000    
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO 0    
Employer contributions 0 $ 0  
Other Postretirement Benefits | PNM and TNMP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer contributions 0 0  
Expected employer contributions in fiscal year through year 5 0 0  
Executive Retirement Program | Public Service Company of New Mexico      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarial gains (losses) recorded as regulatory assets (78,000)    
Expected disbursements by employer 1,400,000 1,400,000  
Executive Retirement Program | Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarial gains (losses) recorded as regulatory assets (58,000)    
Expected disbursements by employer $ 100,000 $ 100,000  
Non-US | Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 13.00%