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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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 | 4.00% |  |
Pension Plan | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Weighted average discount rate related to anticipated contributions | 5.10% |  |
Pension Plan | Equity Securities |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 21.00% |  |
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 | 6.00% |  |
Pension Plan | Fixed income |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Investment allocation targets | 65.00% |  |
Pension Plan | Fixed income | Scenario, plan |  |  |
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 | 14.00% |  |
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 | 6.40% | 6.50% | 6.80%
Expected long-term return on assets decrease resulting in increase net periodic costs In next fiscal year | $ 5,300,000 |  |
Actual rate of return for pension plans | 13.40% |  |
Expected employer contributions in year 5 | $ 5,100,000 |  |
Actuarial gains (losses) recorded as regulatory assets | $ 9,288,000 |  |
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 | 6.40% | 6.50% | 6.80%
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 | 12.80% |  |
Expected employer contributions in year 5 | $ 0 |  |
Actuarial gains (losses) recorded as regulatory assets | $ 621,000 |  |
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 | 7.50% | 7.70% | 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 | 20.50% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 10,600,000 |  |
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 | 5.40% | 5.70% | 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 | 19.40% |  |
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 |  |
Estimated future employer contributions in next fiscal year | 300,000 |  |
Estimated future employer contributions in years two through five | $ 1,400,000 |  |

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