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Pension and Other Postretirement Benefit Plans (Details) - USD ($)
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Other (deductions) $ 1,376,000 $ 5,621,000  
Regulatory assets 595,398,000   $ 600,672,000
Other income 3,469,000 4,902,000  
Regulatory liabilities 934,053,000   933,578,000
PNM      
Defined Benefit Plan Disclosure [Line Items]      
Other (deductions) 1,462,000 4,959,000  
Regulatory assets 453,912,000   459,239,000
Other income 2,391,000 3,843,000  
Regulatory liabilities 749,319,000   754,441,000
PNM | Pension Plan      
Components of Net Periodic Benefit Cost      
Service cost 0 0  
Interest cost 6,068,000 6,727,000  
Expected return on plan assets (8,672,000) (8,451,000)  
Amortization of net (gain) loss 4,087,000 4,001,000  
Amortization of prior service cost (241,000) (241,000)  
Net Periodic Benefit Cost 1,242,000 2,036,000  
Contributions by employer 0 0  
Total expected employer contributions for future fiscal years 0    
Expected employer contributions in year 5 $ 5,100,000    
PNM | Pension Plan | Minimum      
Components of Net Periodic Benefit Cost      
Assumptions used calculating net periodic benefit cost, discount rate 4.00%    
PNM | Pension Plan | Maximum      
Components of Net Periodic Benefit Cost      
Assumptions used calculating net periodic benefit cost, discount rate 5.10%    
PNM | OPEB Plan      
Components of Net Periodic Benefit Cost      
Service cost $ 21,000 24,000  
Interest cost 860,000 1,006,000  
Expected return on plan assets (1,353,000) (1,308,000)  
Amortization of net (gain) loss 588,000 921,000  
Amortization of prior service cost (416,000) (416,000)  
Net Periodic Benefit Cost (300,000) 227,000  
Contributions by employer 0    
Estimated employer contributions in Year 2-5 0    
PNM | Executive Retirement Program      
Components of Net Periodic Benefit Cost      
Service cost 0 0  
Interest cost 155,000 174,000  
Expected return on plan assets 0 0  
Amortization of net (gain) loss 90,000 78,000  
Amortization of prior service cost 0 0  
Net Periodic Benefit Cost 245,000 252,000  
Contributions by employer 500,000 500,000  
Total expected employer contributions for fiscal year 1,500,000    
Estimated employer contributions in Year 2-5 5,700,000    
TNMP      
Defined Benefit Plan Disclosure [Line Items]      
Other (deductions) (331,000) 90,000  
Regulatory assets 141,486,000   141,433,000
Other income 754,000 822,000  
Regulatory liabilities 184,734,000   $ 179,137,000
TNMP | Pension Plan      
Components of Net Periodic Benefit Cost      
Service cost 0 0  
Interest cost 656,000 722,000  
Expected return on plan assets (991,000) (945,000)  
Amortization of net (gain) loss 272,000 231,000  
Amortization of prior service cost 0 0  
Net Periodic Benefit Cost (63,000) 8,000  
Contributions by employer 0 0  
Estimated employer contributions in Year 2-5 $ 0    
TNMP | Pension Plan | Minimum      
Components of Net Periodic Benefit Cost      
Assumptions used calculating net periodic benefit cost, discount rate 4.00%    
TNMP | Pension Plan | Maximum      
Components of Net Periodic Benefit Cost      
Assumptions used calculating net periodic benefit cost, discount rate 5.10%    
TNMP | OPEB Plan      
Components of Net Periodic Benefit Cost      
Service cost $ 33,000 36,000  
Interest cost 119,000 139,000  
Expected return on plan assets (135,000) (114,000)  
Amortization of net (gain) loss (56,000) (20,000)  
Amortization of prior service cost 0 0  
Net Periodic Benefit Cost (39,000) 41,000  
Contributions by employer 300,000 700,000  
Estimated employer contributions in Year 2-5 1,400,000    
TNMP | Executive Retirement Program      
Components of Net Periodic Benefit Cost      
Service cost 0 0  
Interest cost 7,000 8,000  
Expected return on plan assets 0 0  
Amortization of net (gain) loss 4,000 2,000  
Amortization of prior service cost 0 0  
Net Periodic Benefit Cost 11,000 10,000  
Total expected employer contributions for fiscal year 100,000    
Estimated employer contributions in Year 2-5 400,000    
TNMP | Executive Retirement Program | Maximum      
Components of Net Periodic Benefit Cost      
Contributions by employer 100,000 100,000  
Accounting Standards Update 2017-07      
Defined Benefit Plan Disclosure [Line Items]      
Other (deductions)   $ 2,100,000  
Accounting Standards Update 2017-07 | PNM      
Defined Benefit Plan Disclosure [Line Items]      
Other (deductions) 900,000    
Regulatory assets 300,000    
Accounting Standards Update 2017-07 | TNMP      
Defined Benefit Plan Disclosure [Line Items]      
Other income 100,000    
Regulatory liabilities $ 100,000