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Pension and Other Postretirement Benefits - APBO, PBO, Fair Value of Plan Assets, and Funded Status of the Plans (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Non-current liability | $ 94,003 | $ 125,844 |
Public Service Company of New Mexico |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Non-current liability | 86,124 | 114,427 |
Public Service Company of New Mexico | Pension Plan |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 621,751 | 597,900 |
Service cost | 0 | 0 | $ 0
Interest cost | 26,908 | 30,307 | 28,255
Actuarial (gain) loss | 26,298 | 39,463 |
Benefits paid | (50,974) | (45,919) |
Balance at end of year | 623,983 | 621,751 | 597,900
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets at beginning of year | 543,601 | 557,923 |
Actual return on plan assets | 69,389 | 31,597 |
Employer contributions | 0 | 0 |
Fair value of plan assets at end of year | 562,016 | 543,601 | 557,923
Funded status – asset (liability) for pension benefits | (61,967) | (78,150) |
Public Service Company of New Mexico | Other Postretirement Benefits |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 94,269 | 84,674 |
Service cost | 96 | 140 | 204
Interest cost | 4,025 | 4,346 | 4,089
Participant contributions | 3,069 | 2,690 |
Actuarial (gain) loss | (1,601) | 17,877 |
Benefits paid | (9,961) | (11,734) |
Plan design changes | 0 | (3,724) |
Balance at end of year | 89,897 | 94,269 | 84,674
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets at beginning of year | 72,694 | 72,952 |
Actual return on plan assets | 14,222 | 5,923 |
Employer contributions | 332 | 2,863 |
Fair value of plan assets at end of year | 80,356 | 72,694 | 72,952
Funded status – asset (liability) for pension benefits | (9,541) | (21,575) |
Public Service Company of New Mexico | Executive Retirement Program |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 16,212 | 16,105 |
Service cost | 0 | 0 | 0
Interest cost | 697 | 812 | 760
Actuarial (gain) loss | 674 | 768 |
Benefits paid | (1,466) | (1,473) |
Balance at end of year | 16,117 | 16,212 | 16,105
Less current liability | 1,501 | 1,510 |
Non-current liability | 14,616 | 14,702 |
Texas-New Mexico Power Company |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Non-current liability | 7,879 | 11,417 |
Texas-New Mexico Power Company | Pension Plan |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 67,061 | 64,198 |
Service cost | 0 | 0 | 0
Interest cost | 2,887 | 3,304 | 3,043
Actuarial (gain) loss | 3,050 | 4,318 |
Benefits paid | (4,575) | (4,759) |
Balance at end of year | 68,423 | 67,061 | 64,198
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets at beginning of year | 60,624 | 62,082 |
Actual return on plan assets | 7,450 | 3,301 |
Employer contributions | 0 | 0 |
Fair value of plan assets at end of year | 63,499 | 60,624 | 62,082
Funded status – asset (liability) for pension benefits | (4,924) | (6,437) |
Texas-New Mexico Power Company | Other Postretirement Benefits |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 12,830 | 13,106 |
Service cost | 143 | 186 | 247
Interest cost | 556 | 677 | 608
Participant contributions | 379 | 520 |
Actuarial (gain) loss | (381) | (96) |
Benefits paid | (1,248) | (1,563) |
Plan design changes | 0 | 0 |
Balance at end of year | 12,279 | 12,830 | 13,106
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |  |
Fair value of plan assets at beginning of year | 8,544 | 9,111 |
Actual return on plan assets | 1,642 | 476 |
Employer contributions | 685 | 0 |
Fair value of plan assets at end of year | 10,002 | 8,544 | 9,111
Funded status – asset (liability) for pension benefits | (2,277) | (4,286) |
Texas-New Mexico Power Company | Executive Retirement Program |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Balance at beginning of year | 787 | 794 |
Service cost | 0 | 0 | 0
Interest cost | 33 | 40 | 36
Actuarial (gain) loss | 44 | 47 |
Benefits paid | (93) | (94) |
Balance at end of year | 771 | 787 | $ 794
Less current liability | 93 | 93 |
Non-current liability | $ 678 | $ 694 |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + References Reference 1: 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=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contributions received by defined benefit plan from participant which increase benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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