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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) Recognized (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Public Service Company of New Mexico | Pension Plan |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | $ 0 | $ 0 | $ 0
Interest cost | 26,908 | 30,307 | 28,255
Expected return on plan assets | (33,803) | (35,416) | (39,323)
Amortization of net (gain) loss | 16,006 | 13,820 | 14,820
Amortization of prior service cost | (965) | (965) | (965)
Net periodic benefit cost | 8,146 | 7,746 | 2,787
Public Service Company of New Mexico | Other Postretirement Benefits |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | 96 | 140 | 204
Interest cost | 4,025 | 4,346 | 4,089
Expected return on plan assets | (5,230) | (5,483) | (5,610)
Amortization of net (gain) loss | 3,682 | 1,145 | 1,966
Amortization of prior service cost | (1,663) | (30) | (642)
Net periodic benefit cost | 910 | 118 | 7
Public Service Company of New Mexico | Executive Retirement Program |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | 0 | 0 | 0
Interest cost | 697 | 812 | 760
Amortization of net (gain) loss | 313 | 256 | 325
Amortization of prior service cost | 0 | 0 | 0
Net periodic benefit cost | 1,010 | 1,068 | 1,085
Texas-New Mexico Power Company |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Net periodic benefit cost | 100 | 100 |
Texas-New Mexico Power Company | Pension Plan |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | 0 | 0 | 0
Interest cost | 2,887 | 3,304 | 3,043
Expected return on plan assets | (3,779) | (3,943) | (4,420)
Amortization of net (gain) loss | 923 | 700 | 782
Amortization of prior service cost | 0 | 0 | 0
Net periodic benefit cost | 31 | 61 | (595)
Texas-New Mexico Power Company | Other Postretirement Benefits |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | 143 | 186 | 247
Interest cost | 556 | 677 | 608
Expected return on plan assets | (456) | (490) | (520)
Amortization of net (gain) loss | (79) | (40) | 0
Amortization of prior service cost | 0 | 0 | 0
Net periodic benefit cost | 164 | 333 | 335
Texas-New Mexico Power Company | Executive Retirement Program |  |  |
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] |  |  |
Service cost | 0 | 0 | 0
Interest cost | 33 | 40 | 36
Amortization of net (gain) loss | 9 | 2 | 5
Amortization of prior service cost | 0 | 0 | 0
Net periodic benefit cost | $ 42 | $ 42 | $ 41

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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit 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 (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 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)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of net periodic benefit cost (credit) for defined benefit 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 (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 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)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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