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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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 | 2.90% |  |
Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 35.00% |  |
Pension Plan | Fixed income |  |  |
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 | 15.00% |  |
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 | 5.90% | 6.86% | 6.54%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 5,200,000 |  |
Rate of return for retirement plan | 19.30% |  |
Expected employer contributions in year two | $ 10,800,000 |  |
Expected employer contributions in year 3 | 11,500,000 |  |
Expected employer contributions in year 4 | 10,600,000 |  |
Actuarial gains (losses) recorded as regulatory assets | 21,393,000 |  |
Employer contributions | $ 0 | $ 0 |
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 | 5.90% | 6.90% | 6.57%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 600,000 |  |
Rate of return for retirement plan | 20.40% |  |
Expected employer contributions in year two | $ 0 |  |
Expected employer contributions in year 3 | 0 |  |
Expected employer contributions in year 4 | 0 |  |
Actuarial gains (losses) recorded as regulatory assets | (3,860,000) |  |
Employer contributions | $ 0 | $ 0 |
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 | 30.00% |  |
Other Postretirement Benefits | Fixed income |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 70.00% |  |
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 | 7.00% | 7.20% | 7.42%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 900,000 |  |
Rate of return for retirement plan | 11.10% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 600,000 |  |
Employer contributions | 3,256,000 | $ 3,580,000 |
Expected employer disbursements for next fiscal year | 3,700,000 |  |
Expected employer disbursements by employer in year 2 through year 5 | $ 13,100,000 |  |
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 | 5.60% | 5.80% | 5.86%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 100,000 |  |
Rate of return for retirement plan | 23.60% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 1,200,000 |  |
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO | 0 |  |
Employer contributions | 0 | $ 0 |
Other Postretirement Benefits | PNM and TNMP |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer contributions | 0 | 0 |
Expected employer contributions in fiscal year through year 5 | 0 | 0 |
Executive Retirement Program | Public Service Company of New Mexico |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Actuarial gains (losses) recorded as regulatory assets | (78,000) |  |
Expected disbursements by employer | 1,400,000 | 1,400,000 |
Executive Retirement Program | Texas-New Mexico Power Company |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Actuarial gains (losses) recorded as regulatory assets | (58,000) |  |
Expected disbursements by employer | $ 100,000 | $ 100,000 |
Non-US | Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 13.00% |  |

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