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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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 $ 15,696,000 $ 24,247,000 $ 20,524,000
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 $ 11,128,000 18,556,000 16,279,000
Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses $ 1,422,000 $ 1,443,000 $ 1,427,000
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 4.20%    
Pension Plan | Maximum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Weighted average discount rate related to anticipated contributions 4.60%    
Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 26.00%    
Pension Plan | Equity Securities | Developed Countries Outside of United States      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 7.00%    
Pension Plan | Fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 54.00%    
Pension Plan | Fixed income | Scenario, plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 54.00%    
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 6.54% 6.40% 6.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 5,000,000    
Actual rate of return for pension plans (5.40%)    
Expected employer contributions in year 5 $ 1,300,000    
Actuarial gains (losses) recorded as regulatory assets $ (22,728,000)    
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 6.57% 6.40% 6.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 600,000    
Actual rate of return for pension plans (5.20%)    
Expected employer contributions in year 5 $ 0    
Actuarial gains (losses) recorded as regulatory assets $ (1,926,000)    
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 7.42% 7.50% 7.70%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 700,000    
Actual rate of return for pension plans (9.70%)    
Actuarial gains (losses) recorded as regulatory assets $ 900,000    
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 5.86% 5.40% 5.70%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 100,000    
Actual rate of return for pension plans (10.00%)    
Actuarial gains (losses) recorded as regulatory assets $ 1,600,000    
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO 0    
Accounting Standards Update 2017-07      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses   $ 8,600,000 $ 6,700,000
Accounting Standards Update 2017-07 | Public Service Company of New Mexico      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses 4,300,000    
Non-service cost deferred as regulatory assets 400,000    
Accounting Standards Update 2017-07 | Texas-New Mexico Power Company      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Other expenses 300,000    
Non-service cost deferred as regulatory liability $ 100,000