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Pension and Other Postretirement Benefit Plans (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
PNM | Pension Plan        
Components of Net Periodic Benefit Cost        
Service cost $ 0 $ 0 $ 0 $ 0
Interest cost 4,035,000 4,985,000 8,071,000 9,971,000
Expected return on plan assets (7,132,000) (7,363,000) (14,265,000) (14,726,000)
Amortization of net loss 4,542,000 4,465,000 9,083,000 8,930,000
Amortization of prior service cost 0 (138,000) 0 (277,000)
Net Periodic Benefit Cost (Income) 1,445,000 1,949,000 2,889,000 3,898,000
Contributions by employer     0 0
Expected employer contributions, remainder of fiscal year 0   0  
Expected employer contributions in year 2 10,800,000   10,800,000  
Expected employer contributions in year 3 11,500,000   11,500,000  
Expected employer contributions in year 4 10,600,000   $ 10,600,000  
PNM | Pension Plan | Maximum        
Components of Net Periodic Benefit Cost        
Assumptions used calculating net periodic benefit cost, discount rate     2.90%  
PNM | OPEB Plan        
Components of Net Periodic Benefit Cost        
Service cost 7,000 10,000 $ 13,000 20,000
Interest cost 477,000 613,000 954,000 1,227,000
Expected return on plan assets (1,041,000) (1,387,000) (2,083,000) (2,774,000)
Amortization of net loss 0 87,000 0 174,000
Amortization of prior service cost 0 0 0
Net Periodic Benefit Cost (Income) (557,000) (677,000) (1,116,000) (1,353,000)
Contributions by employer     0 0
Disbursements by employer 800,000 1,100,000 1,700,000 2,100,000
Expected employer disbursements for remainder of fiscal year 3,300,000   3,300,000  
Expected employer disbursements in year 2 through 5 13,100,000   13,100,000  
PNM | Executive Retirement Program        
Components of Net Periodic Benefit Cost        
Service cost 0 0 0 0
Interest cost 91,000 122,000 181,000 244,000
Expected return on plan assets 0 0 0 0
Amortization of net loss 97,000 101,000 197,000 201,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) 188,000 223,000 378,000 445,000
Disbursements by employer 300,000 300,000 800,000 800,000
Expected employer disbursements for remainder of fiscal year 1,300,000   1,300,000  
Expected employer disbursements in year 2 through 5 4,900,000   4,900,000  
TNMP | Pension Plan        
Components of Net Periodic Benefit Cost        
Service cost 0 0 0 0
Interest cost 436,000 544,000 871,000 1,088,000
Expected return on plan assets (796,000) (821,000) (1,591,000) (1,642,000)
Amortization of net loss 312,000 315,000 624,000 629,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) (48,000) 38,000 (96,000) 75,000
Contributions by employer     0 0
Estimated employer contributions for remainder of fiscal year through year 5 0   $ 0  
TNMP | Pension Plan | Maximum        
Components of Net Periodic Benefit Cost        
Assumptions used calculating net periodic benefit cost, discount rate     2.90%  
TNMP | OPEB Plan        
Components of Net Periodic Benefit Cost        
Service cost 11,000 12,000 $ 22,000 23,000
Interest cost 77,000 93,000 154,000 187,000
Expected return on plan assets (101,000) (135,000) (203,000) (268,000)
Amortization of net loss (81,000) (80,000) (161,000) (162,000)
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) (94,000) (110,000) (188,000) (220,000)
Contributions by employer 0 0 0 0
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 5,000 6,000 9,000 11,000
Expected return on plan assets 0 0 0 0
Amortization of net loss 8,000 6,000 17,000 12,000
Amortization of prior service cost 0 0 0 0
Net Periodic Benefit Cost (Income) 13,000 12,000 26,000 23,000
Expected employer disbursements for remainder of fiscal year 100,000   100,000  
Expected employer disbursements in year 2 through 5 300,000   300,000  
TNMP | Executive Retirement Program | Maximum        
Components of Net Periodic Benefit Cost        
Disbursements by employer $ 0 $ 0 $ 0 $ 0