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Summary of the Business and Significant Accounting Policies - Narrative (Details) |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
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Mar. 01, 2018 USD ($) | Dec. 31, 2017 USD ($) Facility MW | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) Facility MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jun. 30, 2018 MW | Sep. 29, 2017 | Sep. 22, 2017 | Jul. 08, 2016 MW | Dec. 31, 2014 USD ($) | Dec. 31, 1986 lease
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment defaults |  | $ 0 |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  |  | $ 9,516,000 | $ 4,949,000 | $ 10,430,000 |  |  |  |  |  |
Weighted average borrowing rate |  | 5.90% |  |  |  | 6.10% |  |  |  | 5.90% | 6.10% | 6.60% |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  |  | $ 1,300,000 | $ 1,800,000 | $ 1,500,000 |  |  |  |  |  |
Plant in Service |  | $ 7,238,285,000 |  |  |  | $ 6,944,534,000 |  |  |  | 7,238,285,000 | 6,944,534,000 |  |  |  |  |  |  |
Operating Expenses (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 1,138,518,000 | 1,084,966,000 | 1,314,732,000 |  |  |  |  |  |
Net income (less than $0.1 million) |  | (54,282,000) | $ 73,739,000 | $ 37,555,000 | $ 22,862,000 | 24,809,000 | $ 54,418,000 | $ 27,076,000 | $ 10,546,000 | 79,874,000 | 116,849,000 | 15,640,000 |  |  |  |  |  |
Construction work in progress |  | 245,933,000 |  |  |  | 208,206,000 |  |  |  | 245,933,000 | 208,206,000 |  |  |  |  |  |  |
Owner's equity |  | 1,695,253,000 |  |  |  | 1,675,952,000 |  |  |  | 1,695,253,000 | 1,675,952,000 |  |  |  |  |  |  |
Unrealized gains, net of income taxes, recorded in AOCI |  | 11,100,000 |  |  |  |  |  |  |  | 11,100,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for funds used during construction, capitalized interest |  |  |  |  |  |  |  |  |  | 6,300,000 | 5,300,000 | 7,800,000 |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  |  | 8,664,000 | 4,163,000 | 10,430,000 |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  |  | 600,000 | 800,000 | 800,000 |  |  |  |  |  |
Impairment losses on securities held in the NDT |  |  |  |  |  |  |  |  |  | 7,100,000 | 13,900,000 | 10,400,000 |  |  |  |  |  |
Plant in Service |  | 5,501,070,000 |  |  |  | 5,359,211,000 |  |  |  | 5,501,070,000 | 5,359,211,000 |  |  |  |  |  |  |
Operating Expenses (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 891,705,000 | 847,823,000 | 1,097,815,000 |  |  |  |  |  |
Net income (less than $0.1 million) |  | (32,021,000) | 60,827,000 | 26,932,000 | 16,658,000 | 16,112,000 | 40,984,000 | 16,049,000 | 4,274,000 | 72,396,000 | 77,419,000 | (15,234,000) |  |  |  |  |  |
Construction work in progress |  | 204,079,000 |  |  |  | 158,122,000 |  |  |  | 204,079,000 | 158,122,000 |  |  |  |  |  |  |
Owner's equity |  | $ 1,422,174,000 |  |  |  | 1,397,872,000 |  |  |  | 1,422,174,000 | 1,397,872,000 |  |  |  |  |  |  |
Restricted cash deposits |  |  |  |  |  |  |  |  |  |  |  | 8,200,000 |  |  |  |  |  |
Administrative and general, pre-tax |  |  |  |  |  |  |  |  |  | $ 4,700,000 | 3,000,000 |  |  |  |  |  |  |
Public Service Company of New Mexico | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Refund to third party | $ 7,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 10.3% Lessor Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes receivable, stated percentage rate |  | 10.30% |  |  |  |  |  |  |  | 10.30% |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 10.15% Lessor Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes receivable, stated percentage rate |  | 10.15% |  |  |  |  |  |  |  | 10.15% |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of operating leases | lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for funds used during construction, capitalized interest |  |  |  |  |  |  |  |  |  | $ 1,200,000 | 900,000 | 500,000 |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  |  | 900,000 | 800,000 | 0 |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  |  | 100,000 | 100,000 | 100,000 |  |  |  |  |  |
Plant in Service |  | $ 1,504,778,000 |  |  |  | 1,380,584,000 |  |  |  | 1,504,778,000 | 1,380,584,000 |  |  |  |  |  |  |
Operating Expenses (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 247,169,000 | 235,397,000 | 217,854,000 |  |  |  |  |  |
Net income (less than $0.1 million) |  | 1,024,000 | $ 14,727,000 | $ 12,204,000 | $ 7,604,000 | 9,855,000 | $ 13,853,000 | $ 10,508,000 | $ 7,456,000 | 35,559,000 | 41,672,000 | 41,963,000 |  |  |  |  |  |
Construction work in progress |  | 34,350,000 |  |  |  | 16,978,000 |  |  |  | 34,350,000 | 16,978,000 |  |  |  |  |  |  |
Owner's equity |  | 634,405,000 |  |  |  | $ 593,235,000 |  |  |  | 634,405,000 | 593,235,000 | $ 533,380,000 |  |  |  |  | $ 524,665,000 |
Net periodic benefit cost (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 100,000 | $ 100,000 |  |  |  |  |  |  |
PNMR Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Solar distributed generation (in mw) | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 |  |
Solar capacity (in mw) | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 |  |
NMRD |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 100,000 |  |  |  |  |  |  |  |
Operating Expenses (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 100,000 |  |  |  |  |  |  |  |
Net income (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 100,000 |  |  |  |  |  |  |  |
Cash |  | 6,000,000 |  |  |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  |
Construction work in progress |  | 30,900,000 |  |  |  |  |  |  |  | 30,900,000 |  |  |  |  |  |  |  |
Accounts payable |  | 3,900,000 |  |  |  |  |  |  |  | 3,900,000 |  |  |  |  |  |  |  |
Owner's equity |  | $ 33,000,000 |  |  |  |  |  |  |  | $ 33,000,000 |  |  |  |  |  |  |  |
NMRD |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Solar distributed generation (in mw) | MW |  | 30 |  |  |  |  |  |  |  | 30 |  |  |  |  |  |  |  |
Plant in Service |  | $ 12,400,000 |  |  |  |  |  |  |  | $ 12,400,000 |  |  |  |  |  |  |  |
Renewable energy capacity under contract | MW |  | 31.8 |  |  |  |  |  |  |  | 31.8 |  |  |  |  |  |  |  |
Megawatts for solar PV facilities in operation (in mw) | MW |  | 11.8 |  |  |  |  |  |  |  | 11.8 |  |  |  |  |  |  |  |
Megawatts supplying energy to data center (in mw) | MW |  | 10 |  |  |  |  |  |  |  | 10 |  |  |  |  |  |  |  |
Megawatts supplying energy to Columbus Electric Cooperative (in mw) | MW |  | 1.8 |  |  |  |  |  |  |  | 1.8 |  |  |  |  |  |  |  |
Solar capacity (in mw) | MW |  | 20 |  |  |  |  |  |  |  | 20 |  |  |  |  |  |  |  |
NMRD | PNMR Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |
Number of solar facilities | Facility |  | 3 |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |
Solar distributed generation (in mw) | MW |  | 10 |  |  |  |  |  |  |  | 10 |  |  |  |  |  |  |  |
Plant in Service |  | $ 24,800,000 |  |  |  |  |  |  |  | $ 24,800,000 |  |  |  |  |  |  |  |
Cash contribution percentage |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |
NMRD | AEP OnSite Partners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
NMRD | PNMR Development and AEP OnSite |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contribution to construction activities |  |  |  |  |  |  |  |  |  | $ 4,100,000 |  |  |  |  |  |  |  |
Scenario, forecast | NMRD |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Solar distributed generation (in mw) | MW |  |  |  |  |  |  |  |  |  |  |  |  | 20 |  |  |  |  |

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