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Pension and Other Postretirement Benefit Plans (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
PNM | Pension Plan        
Components of Net Periodic Benefit Cost        
Service cost $ 0 $ 0 $ 0 $ 0
Interest cost 6,294,000 6,068,000 18,881,000 18,203,000
Expected return on plan assets (8,527,000) (8,672,000) (25,577,000) (26,014,000)
Amortization of net (gain) loss 3,880,000 4,087,000 11,639,000 12,261,000
Amortization of prior service cost (241,000) (241,000) (724,000) (724,000)
Net Periodic Benefit Cost (Income) 1,406,000 1,242,000 4,219,000 3,726,000
Contributions by employer     0 0
Expected employer contributions, remainder of fiscal year 0   0  
Estimated employer contributions in next fiscal year 0   0  
Expected employer contributions in year 3 0   0  
Expected employer contributions in year 4 23,000,000.0   23,000,000.0  
Expected employer contributions in year 5 27,400,000   $ 27,400,000  
Assumptions used calculating net periodic benefit cost, discount rate     3.50%  
PNM | OPEB Plan        
Components of Net Periodic Benefit Cost        
Service cost 13,000 21,000 $ 39,000 62,000
Interest cost 829,000 860,000 2,487,000 2,579,000
Expected return on plan assets (1,320,000) (1,353,000) (3,956,000) (4,061,000)
Amortization of net (gain) loss 169,000 588,000 507,000 1,765,000
Amortization of prior service cost (99,000) (416,000) (297,000) (1,248,000)
Net Periodic Benefit Cost (Income) (408,000) (300,000) (1,220,000) (903,000)
Contributions by employer 900,000 0 2,400,000 0
Disbursements by employer   900,000   1,500,000
Estimated future employer disbursements, remainder of fiscal year 3,700,000   3,700,000  
Expected employer disbursements in year 2 through 5 13,700,000   13,700,000  
Estimated employer contributions for remainder of fiscal year through year 5 0   0  
PNM | Executive Retirement Program        
Components of Net Periodic Benefit Cost        
Service cost 0 0 0 0
Interest cost 163,000 155,000 487,000 467,000
Expected return on plan assets 0 0 0 0
Amortization of net (gain) loss 79,000 90,000 237,000 269,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) 242,000 245,000 724,000 736,000
Disbursements by employer 400,000 400,000 1,100,000 1,300,000
Expected employer disbursements in year 2 through 5 5,600,000   5,600,000  
Total expected employer disbursements for remainder of fiscal year 1,500,000   1,500,000  
TNMP | Pension Plan        
Components of Net Periodic Benefit Cost        
Service cost 0 0 0 0
Interest cost 672,000 656,000 2,015,000 1,968,000
Expected return on plan assets (967,000) (991,000) (2,901,000) (2,972,000)
Amortization of net (gain) loss 235,000 272,000 706,000 816,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) (60,000) (63,000) (180,000) (188,000)
Contributions by employer     0 0
Expected employer contributions, remainder of fiscal year 0   0  
Estimated employer contributions in next fiscal year 0   0  
Expected employer contributions in year 3 0   0  
Expected employer contributions in year 4 1,000,000.0   1,000,000.0  
Expected employer contributions in year 5 3,700,000   $ 3,700,000  
Assumptions used calculating net periodic benefit cost, discount rate     3.50%  
TNMP | OPEB Plan        
Components of Net Periodic Benefit Cost        
Service cost 12,000 33,000 $ 39,000 100,000
Interest cost 113,000 119,000 338,000 358,000
Expected return on plan assets (129,000) (135,000) (387,000) (406,000)
Amortization of net (gain) loss (111,000) (56,000) (332,000) (170,000)
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) (115,000) (39,000) (342,000) (118,000)
Contributions by employer 0 0 0 300,000
Estimated employer contributions for remainder of fiscal year through year 5 0   0  
TNMP | Executive Retirement Program        
Components of Net Periodic Benefit Cost        
Service cost 0 0 0 0
Interest cost 7,000 7,000 24,000 22,000
Expected return on plan assets 0 0 0 0
Amortization of net (gain) loss 4,000 4,000 11,000 11,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) 11,000 11,000 35,000 33,000
Expected employer disbursements in year 2 through 5 300,000   300,000  
Total expected employer disbursements for remainder of fiscal year 100,000   100,000  
TNMP | Executive Retirement Program | Maximum        
Components of Net Periodic Benefit Cost        
Disbursements by employer $ 100,000 $ 100,000 $ 100,000 $ 100,000