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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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 4.00%    
Pension Plan | Maximum      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Weighted average discount rate related to anticipated contributions 5.10%    
Pension Plan | Equity Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 21.00%    
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 6.00%    
Pension Plan | Fixed income      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Investment allocation targets 65.00%    
Pension Plan | Fixed income | Scenario, plan      
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 14.00%    
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 6.40% 6.50% 6.80%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year $ 5,300,000    
Actual rate of return for pension plans 13.40%    
Expected employer contributions in year 5 $ 5,100,000    
Actuarial gains (losses) recorded as regulatory assets $ 9,288,000    
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 6.40% 6.50% 6.80%
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 12.80%    
Expected employer contributions in year 5 $ 0    
Actuarial gains (losses) recorded as regulatory assets $ 621,000    
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 7.50% 7.70% 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 20.50%    
Actuarial gains (losses) recorded as regulatory assets $ 10,600,000    
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 5.40% 5.70% 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 19.40%    
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    
Estimated future employer contributions in next fiscal year 300,000    
Estimated future employer contributions in years two through five $ 1,400,000