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Pension and Other Postretirement Benefits - Narrative (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
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 [Member] |  |  |
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 [Member] |  |  |
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 [Member] |  |  |
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 [Member] | Minimum [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Weighted average discount rate related to anticipated contributions | 4.10% |  |
Pension Plan [Member] | Maximum [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Weighted average discount rate related to anticipated contributions | 4.90% |  |
Pension Plan [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 21.00% |  |
Pension Plan [Member] | Equity Securities [Member] | Developed Countries Outside of United States [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 6.00% |  |
Pension Plan [Member] | Fixed income [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 65.00% |  |
Pension Plan [Member] | Alternative Investments [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 14.00% |  |
Pension Plan [Member] | Public Service Company of New Mexico [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 6.50% | 6.80% | 7.20%
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 | 5.90% |  |
Actuarial gains (losses) recorded as regulatory assets | $ (43,283,000) |  |
Pension Plan [Member] | Texas-New Mexico Power Company [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 6.50% | 6.80% | 7.20%
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 | 5.50% |  |
Actuarial gains (losses) recorded as regulatory assets | $ (4,960,000) |  |
Other Postretirement Benefits [Member] |  |  |
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 [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 70.00% |  |
Other Postretirement Benefits [Member] | Fixed income [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target plan asset allocations | 30.00% |  |
Other Postretirement Benefits [Member] | Public Service Company of New Mexico [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 7.70% | 7.70% | 8.50%
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 | 8.50% |  |
Actuarial gains (losses) recorded as regulatory assets | $ (17,400,000) |  |
Other Postretirement Benefits [Member] | Texas-New Mexico Power Company [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 5.70% | 5.70% | 6.50%
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 | 5.50% |  |
Actuarial gains (losses) recorded as regulatory assets | $ 100,000 |  |
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 | 400,000 |  |
Estimated future employer contributions in years two through five | $ 1,400,000 |  |

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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512046 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=66004124&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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