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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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 | $ 15,696,000 | $ 24,247,000 | $ 20,524,000
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 | $ 11,128,000 | 18,556,000 | 16,279,000
Texas-New Mexico Power Company |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses | $ 1,422,000 | $ 1,443,000 | $ 1,427,000
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 | 4.20% |  |
Pension Plan | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Weighted average discount rate related to anticipated contributions | 4.60% |  |
Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 26.00% |  |
Pension Plan | Equity Securities | Developed Countries Outside of United States |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 7.00% |  |
Pension Plan | Fixed income |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 54.00% |  |
Pension Plan | Fixed income | Scenario, plan |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 54.00% |  |
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 | 6.54% | 6.40% | 6.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 5,000,000 |  |
Actual rate of return for pension plans | (5.40%) |  |
Expected employer contributions in year 5 | $ 1,300,000 |  |
Actuarial gains (losses) recorded as regulatory assets | $ (22,728,000) |  |
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 | 6.57% | 6.40% | 6.50%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 600,000 |  |
Actual rate of return for pension plans | (5.20%) |  |
Expected employer contributions in year 5 | $ 0 |  |
Actuarial gains (losses) recorded as regulatory assets | $ (1,926,000) |  |
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 | 7.42% | 7.50% | 7.70%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 700,000 |  |
Actual rate of return for pension plans | (9.70%) |  |
Actuarial gains (losses) recorded as regulatory assets | $ 900,000 |  |
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 | 5.86% | 5.40% | 5.70%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 100,000 |  |
Actual rate of return for pension plans | (10.00%) |  |
Actuarial gains (losses) recorded as regulatory assets | $ 1,600,000 |  |
Effect of 1%-point change in assumed health care cost trend rates on net periodic expense and APBO | 0 |  |
Accounting Standards Update 2017-07 |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses |  | $ 8,600,000 | $ 6,700,000
Accounting Standards Update 2017-07 | Public Service Company of New Mexico |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses | 4,300,000 |  |
Non-service cost deferred as regulatory assets | 400,000 |  |
Accounting Standards Update 2017-07 | Texas-New Mexico Power Company |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Other expenses | 300,000 |  |
Non-service cost deferred as regulatory liability | $ 100,000 |  |

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