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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 |  |
Other expenses | $ 15,328,000 | $ 15,696,000 | $ 24,247,000
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% |  |
Public Service Company of New Mexico |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses | $ 11,813,000 | 11,128,000 | 18,556,000
Texas-New Mexico Power Company |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses | $ 1,428,000 | $ 1,422,000 | $ 1,443,000
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 | 3.40% |  |
Pension Plan | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Weighted average discount rate related to anticipated contributions | 3.50% |  |
Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 30.00% |  |
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 | 20.00% |  |
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 | 6.86% | 6.54% | 6.40%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 5,000,000.0 |  |
Rate of return for retirement plan | 18.50% |  |
Expected employer contributions in year 4 | $ 4,600,000 |  |
Expected employer contributions in year 5 | 19,100,000 |  |
Expected employer contributions in year 6 | 19,000,000.0 |  |
Actuarial gains (losses) recorded as regulatory assets | (8,926,000) |  |
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 | 6.90% | 6.57% | 6.40%
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 | 18.90% |  |
Expected employer contributions in year 4 | $ 0 |  |
Expected employer contributions in year 5 | 1,100,000 |  |
Expected employer contributions in year 6 | 2,800,000 |  |
Actuarial gains (losses) recorded as regulatory assets | (1,877,000) |  |
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 | 70.00% |  |
Other Postretirement Benefits | Fixed income |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 30.00% |  |
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 | 7.20% | 7.42% | 7.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 800,000 |  |
Rate of return for retirement plan | 28.10% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 11,400,000 |  |
Employer contributions | 3,580,000 | $ 2,924,000 |
Expected employer disbursements for next fiscal year | 3,700,000 |  |
Expected employer disbursements by employer in year 3 through year 6 | $ 13,500,000 |  |
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 | 5.80% | 5.86% | 5.40%
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 | 28.40% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 900,000 |  |
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO | 0 |  |
Employer contributions | 0 | $ 343,000 |
Other Postretirement Benefits | PNM and TNMP |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Employer contributions | 0 |  |
Expected employer contributions in fiscal year through year 6 | 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 | (1,053,000) |  |
Expected disbursements by employer | 1,400,000 | 1,500,000 |
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 | (54,000) |  |
Expected disbursements by employer | $ 100,000 | $ 100,000 |
Accounting Standards Update 2017-07 |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses |  |  | $ 8,600,000
Non-US | Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets |  | 7.00% |

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