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Pension and Other Postretirement Benefit Plans (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]          
Other (deductions) $ 5,868,000 $ 5,042,000   $ 7,243,000 $ 10,663,000
Other income 4,796,000 3,450,000   8,265,000 8,351,000
PNM          
Defined Benefit Plan Disclosure [Line Items]          
Other (deductions) 3,768,000 4,566,000   5,229,000 9,526,000
Other income 2,292,000 2,665,000   4,684,000 6,508,000
PNM | Pension Plan          
Components of Net Periodic Benefit Cost          
Service cost 0 0   0 0
Interest cost 6,068,000 6,727,000   12,135,000 13,454,000
Expected return on plan assets (8,672,000) (8,451,000)   (17,343,000) (16,901,000)
Amortization of net (gain) loss 4,087,000 4,001,000   8,174,000 8,003,000
Amortization of prior service cost (241,000) (241,000)   (483,000) (483,000)
Net Periodic Benefit Cost 1,242,000 2,036,000   2,483,000 4,073,000
Contributions by employer       0 0
Total expected employer contributions for future fiscal years 0     0  
Expected employer contributions in year 5 5,500,000     $ 5,500,000  
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   $ 41,000 48,000
Interest cost 860,000 1,006,000   1,720,000 2,013,000
Expected return on plan assets (1,353,000) (1,308,000)   (2,707,000) (2,615,000)
Amortization of net (gain) loss 588,000 921,000   1,177,000 1,841,000
Amortization of prior service cost (416,000) (416,000)   (832,000) (832,000)
Net Periodic Benefit Cost (300,000) 227,000   (601,000) 455,000
Contributions by employer       0 0
PNM | Executive Retirement Program          
Components of Net Periodic Benefit Cost          
Service cost 0 0   0 0
Interest cost 155,000 174,000   311,000 349,000
Expected return on plan assets 0 0   0 0
Amortization of net (gain) loss 90,000 78,000   179,000 157,000
Amortization of prior service cost 0 0   0 0
Net Periodic Benefit Cost 245,000 252,000   490,000 506,000
Contributions by employer 400,000 400,000   900,000 900,000
Total expected employer contributions for fiscal year 1,600,000     1,600,000  
Estimated employer contributions in Year 2-5 5,700,000     5,700,000  
TNMP          
Defined Benefit Plan Disclosure [Line Items]          
Other (deductions) 1,391,000 109,000   1,060,000 198,000
Other income 2,223,000 541,000   2,976,000 1,363,000
TNMP | Pension Plan          
Components of Net Periodic Benefit Cost          
Service cost 0 0   0 0
Interest cost 656,000 722,000   1,312,000 1,443,000
Expected return on plan assets (991,000) (945,000)   (1,981,000) (1,889,000)
Amortization of net (gain) loss 272,000 231,000   544,000 461,000
Amortization of prior service cost 0 0   0 0
Net Periodic Benefit Cost (63,000) 8,000   (125,000) 15,000
Contributions by employer       0 0
Estimated employer contributions in Year 2-5 0     $ 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   $ 67,000 72,000
Interest cost 119,000 139,000   238,000 278,000
Expected return on plan assets (135,000) (114,000)   (271,000) (228,000)
Amortization of net (gain) loss (56,000) (20,000)   (113,000) (40,000)
Amortization of prior service cost 0 0   0 0
Net Periodic Benefit Cost (39,000) 41,000   (79,000) 82,000
Contributions by employer 0 0   300,000 700,000
Estimated employer contributions in Year 2-5 1,400,000     1,400,000  
TNMP | Executive Retirement Program          
Components of Net Periodic Benefit Cost          
Service cost 0 0   0 0
Interest cost 7,000 8,000   15,000 17,000
Expected return on plan assets 0 0   0 0
Amortization of net (gain) loss 4,000 2,000   8,000 4,000
Amortization of prior service cost 0 0   0 0
Net Periodic Benefit Cost 11,000 10,000   23,000 21,000
Total expected employer contributions for fiscal year 100,000     100,000  
Estimated employer contributions in Year 2-5 400,000     400,000  
TNMP | Executive Retirement Program | Maximum          
Components of Net Periodic Benefit Cost          
Contributions by employer 100,000 100,000   100,000 100,000
Accounting Standards Update 2017-07          
Defined Benefit Plan Disclosure [Line Items]          
Other (deductions)   $ 2,100,000     $ 4,300,000
Accounting Standards Update 2017-07 | PNM          
Defined Benefit Plan Disclosure [Line Items]          
Other (deductions) 1,200,000     2,100,000  
Non-service cost deferred as regulatory assets 100,000     300,000  
Accounting Standards Update 2017-07 | TNMP          
Defined Benefit Plan Disclosure [Line Items]          
Other income 100,000     300,000  
Non-service cost deferred as regulatory liability $ 100,000     $ 100,000  
Forecast | TNMP | OPEB Plan          
Components of Net Periodic Benefit Cost          
Contributions by employer     $ 0