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Summary of the Business and Significant Accounting Policies - Narrative (Details) | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |
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Dec. 31, 2018 USD ($) utility Facility MW | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) utility Facility MW | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jan. 01, 2019 USD ($) | Sep. 29, 2017 | Sep. 22, 2017 | Sep. 05, 2017 MW | Jul. 08, 2016 MW | Dec. 31, 2015 USD ($) | Dec. 31, 1986 lease
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of regulated utilities | utility | 2 |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Payment defaults | $ 0 |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  | $ 10,404,000 | $ 9,516,000 | $ 4,949,000 |  |  |  |  |  |  |
Weighted average borrowing rate | 5.60% |  |  |  | 5.90% |  |  |  | 5.60% | 5.90% | 6.10% |  |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  | $ 600,000 | $ 1,300,000 | $ 1,800,000 |  |  |  |  |  |  |
Plant in Service | $ 7,548,581,000 |  |  |  | $ 7,238,285,000 |  |  |  | 7,548,581,000 | 7,238,285,000 |  |  |  |  |  |  |  |
Revenues (less than $0.1 million) |  |  |  |  |  |  |  |  | 1,436,613,000 | 1,445,003,000 | 1,362,951,000 |  |  |  |  |  |  |
Operating expenses (less than $0.1 million) |  |  |  |  |  |  |  |  | 1,200,566,000 | 1,129,964,000 | 1,078,226,000 |  |  |  |  |  |  |
Net income (less than $0.1 million) | (55,077,000) | $ 87,521,000 | $ 38,208,000 | $ 14,990,000 | (54,282,000) | $ 73,739,000 | $ 37,555,000 | $ 22,862,000 |  |  |  |  |  |  |  |  |  |
Construction work in progress | 194,427,000 |  |  |  | 245,933,000 |  |  |  | 194,427,000 | 245,933,000 |  |  |  |  |  |  |  |
Owner's equity | 1,688,382,000 |  |  |  | 1,695,253,000 |  |  |  | 1,688,382,000 | 1,695,253,000 |  |  |  |  |  |  |  |
Other current assets | $ 53,725,000 |  |  |  | 47,358,000 |  |  |  | $ 53,725,000 | 47,358,000 |  |  |  |  |  |  |  |
PNMR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of regulated utilities | utility | 2 |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash deposits |  |  |  |  |  |  | 1,000,000 |  |  |  | 7,200,000 |  |  |  |  |  | $ 8,200,000 |
Allowance for funds used during construction, capitalized interest |  |  |  |  |  |  |  |  | $ 6,100,000 | 6,300,000 | 5,300,000 |  |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  | 8,173,000 | 8,664,000 | 4,163,000 |  |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  | 200,000 | 600,000 | 800,000 |  |  |  |  |  |  |
Impairment losses on securities held in the NDT |  |  |  |  |  |  |  |  | 13,700,000 | 7,100,000 | 13,900,000 |  |  |  |  |  |  |
Plant in Service | $ 5,623,520,000 |  |  |  | 5,501,070,000 |  |  |  | 5,623,520,000 | 5,501,070,000 |  |  |  |  |  |  |  |
Revenues (less than $0.1 million) |  |  |  |  |  |  |  |  | 1,091,965,000 | 1,104,230,000 | 1,035,913,000 |  |  |  |  |  |  |
Operating expenses (less than $0.1 million) |  |  |  |  |  |  |  |  | 946,932,000 | 883,151,000 | 841,083,000 |  |  |  |  |  |  |
Net income (less than $0.1 million) | (56,806,000) | 77,508,000 | 26,672,000 | 7,837,000 | (32,021,000) | 60,827,000 | 26,932,000 | 16,658,000 | 55,211,000 | 72,396,000 | 77,419,000 |  |  |  |  |  |  |
Construction work in progress | 134,221,000 |  |  |  | 204,079,000 |  |  |  | 134,221,000 | 204,079,000 |  |  |  |  |  |  |  |
Owner's equity | 1,397,359,000 |  |  |  | 1,422,174,000 |  |  |  | 1,397,359,000 | 1,422,174,000 |  |  |  |  |  |  |  |
Other current assets | $ 42,433,000 |  |  |  | 39,904,000 |  |  |  | $ 42,433,000 | 39,904,000 |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 2,300,000 | 1,200,000 | 900,000 |  |  |  |  |  |  |
Allowance for equity funds used during construction |  |  |  |  |  |  |  |  | 2,200,000 | 900,000 | 800,000 |  |  |  |  |  |  |
Interest costs incurred, capitalized |  |  |  |  |  |  |  |  | 100,000 | 100,000 | 100,000 |  |  |  |  |  |  |
Plant in Service | $ 1,686,119,000 |  |  |  | 1,504,778,000 |  |  |  | 1,686,119,000 | 1,504,778,000 |  |  |  |  |  |  |  |
Revenues (less than $0.1 million) |  |  |  |  |  |  |  |  | 344,648,000 | 340,773,000 | 327,038,000 |  |  |  |  |  |  |
Operating expenses (less than $0.1 million) |  |  |  |  |  |  |  |  | 248,151,000 | 247,169,000 | 235,397,000 |  |  |  |  |  |  |
Net income (less than $0.1 million) | 10,711,000 | $ 16,100,000 | $ 15,367,000 | $ 9,413,000 | 1,024,000 | $ 14,727,000 | $ 12,204,000 | $ 7,604,000 | 51,591,000 | 35,559,000 | 41,672,000 |  |  |  |  |  |  |
Construction work in progress | 51,459,000 |  |  |  | 34,350,000 |  |  |  | 51,459,000 | 34,350,000 |  |  |  |  |  |  |  |
Owner's equity | 674,093,000 |  |  |  | 634,405,000 |  |  |  | 674,093,000 | 634,405,000 | 593,235,000 |  |  |  |  |  | 533,380,000 |
Other current assets | 1,114,000 |  |  |  | 1,131,000 |  |  |  | 1,114,000 | 1,131,000 |  |  |  |  |  |  |  |
PNMR Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Solar distributed generation (in mw) | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50 | 30 |  |
NMRD |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues (less than $0.1 million) |  |  |  |  |  |  |  |  | 3,100,000 | 100,000 |  |  |  |  |  |  |  |
Operating expenses (less than $0.1 million) |  |  |  |  |  |  |  |  |  | 100,000 |  |  |  |  |  |  |  |
Net income (less than $0.1 million) |  |  |  |  |  |  |  |  | 1,000,000 | 100,000 |  |  |  |  |  |  |  |
Cash | 2,600,000 |  |  |  | 6,000,000 |  |  |  | 2,600,000 | 6,000,000 |  |  |  |  |  |  |  |
Construction work in progress | 50,800,000 |  |  |  | 30,900,000 |  |  |  | 50,800,000 | 30,900,000 |  |  |  |  |  |  |  |
Accounts payable | 200,000 |  |  |  | 3,900,000 |  |  |  | 200,000 | 3,900,000 |  |  |  |  |  |  |  |
Owner's equity | $ 53,200,000 |  |  |  | 33,000,000 |  |  |  | $ 53,200,000 | 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 | 33.9 |  |  |  |  |  |  |  | 33.9 |  |  |  |  |  |  |  |  |
Megawatts supplying energy to data center (in mw) | MW | 30 |  |  |  |  |  |  |  | 30 |  |  |  |  |  |  |  |  |
Megawatts supplying energy to Columbus Electric Cooperative (in mw) | MW | 3.9 |  |  |  |  |  |  |  | 3.9 |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 9,600,000 | 4,100,000 |  |  |  |  |  |  |  |
Accounting Standards Update 2016-18 | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash, cash equivalents, restricted cash and restricted cash equivalents |  |  |  |  |  |  |  |  |  |  | 1,000,000 |  |  |  |  |  | $ 8,200,000 |
Other current assets |  |  |  |  | 1,000,000 |  |  |  |  | 1,000,000 | $ 7,200,000 |  |  |  |  |  |  |
Accounting Standards Update 2016-18 | NMCUC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash, cash equivalents, restricted cash and restricted cash equivalents |  |  |  |  | $ 100,000 |  |  |  |  | $ 100,000 |  |  |  |  |  |  |  |
Accounting Standards Update 2016-02 | Scenario, forecast | PNMR | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | $ 160,000,000 |  |  |  |  |  |
Operating lease, liability |  |  |  |  |  |  |  |  |  |  |  | 160,000,000 |  |  |  |  |  |
Accounting Standards Update 2016-02 | Scenario, forecast | Public Service Company of New Mexico | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | 146,000,000 |  |  |  |  |  |
Operating lease, liability |  |  |  |  |  |  |  |  |  |  |  | 146,000,000 |  |  |  |  |  |
Accounting Standards Update 2016-02 | Scenario, forecast | Texas-New Mexico Power Company | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  |  |  |  |  |  |  | 12,000,000 |  |  |  |  |  |
Operating lease, liability |  |  |  |  |  |  |  |  |  |  |  | $ 12,000,000 |  |  |  |  |  |

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