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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
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    
Other expenses $ 15,328,000 $ 15,696,000 $ 24,247,000
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%    
Public Service Company of New Mexico      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses $ 11,813,000 11,128,000 18,556,000
Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses $ 1,428,000 $ 1,422,000 $ 1,443,000
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 3.40%    
Pension Plan | Maximum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Weighted average discount rate related to anticipated contributions 3.50%    
Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 30.00%    
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 20.00%    
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 6.86% 6.54% 6.40%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 5,000,000.0    
Rate of return for retirement plan 18.50%    
Expected employer contributions in year 4 $ 4,600,000    
Expected employer contributions in year 5 19,100,000    
Expected employer contributions in year 6 19,000,000.0    
Actuarial gains (losses) recorded as regulatory assets (8,926,000)    
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 6.90% 6.57% 6.40%
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 18.90%    
Expected employer contributions in year 4 $ 0    
Expected employer contributions in year 5 1,100,000    
Expected employer contributions in year 6 2,800,000    
Actuarial gains (losses) recorded as regulatory assets (1,877,000)    
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 70.00%    
Other Postretirement Benefits | Fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 30.00%    
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 7.20% 7.42% 7.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 800,000    
Rate of return for retirement plan 28.10%    
Actuarial gains (losses) recorded as regulatory assets $ 11,400,000    
Employer contributions 3,580,000 $ 2,924,000  
Expected employer disbursements for next fiscal year 3,700,000    
Expected employer disbursements by employer in year 3 through year 6 $ 13,500,000    
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 5.80% 5.86% 5.40%
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 28.40%    
Actuarial gains (losses) recorded as regulatory assets $ 900,000    
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO 0    
Employer contributions 0 $ 343,000  
Other Postretirement Benefits | PNM and TNMP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer contributions 0    
Expected employer contributions in fiscal year through year 6 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 (1,053,000)    
Expected disbursements by employer 1,400,000 1,500,000  
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 (54,000)    
Expected disbursements by employer $ 100,000 $ 100,000  
Accounting Standards Update 2017-07      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses     $ 8,600,000
Non-US | Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets   7.00%