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Financing - Long-term Debt (Details) - USD ($) | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016 | Dec. 21, 2016 | Dec. 17, 2015 | Aug. 11, 2015
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Debt Instrument [Line Items] |  |  |  |  |  |
Total | $ 2,431,507,000 |  | $ 2,388,577,000 |  |  |
Principal, less current maturities | 257,293,000 |  | 274,025,000 |  |  |
Long-term debt, excluding current maturities, gross | 2,174,214,000 |  | 2,114,552,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | (6,138,000) |  | (4,135,000) |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities | 398,000 |  | 677,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities | (6,536,000) |  | (4,812,000) |  |  |
PNMR 2016 Two-Year Term Loan due December 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt |  |  |  | $ 100,000,000.0 |  |
Public Service Company of New Mexico |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total | 1,665,870,000 |  | 1,640,870,000 |  |  |
Principal, less current maturities | 25,000 |  | 232,000,000 |  |  |
Long-term debt, excluding current maturities, gross | 1,665,845,000 |  | 1,408,870,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 7,960,000 |  | 9,501,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities | 2,000 |  | 120,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities | $ 7,958,000 |  | $ 9,381,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 1.875% due April 2033, mandatory tender - October 1, 2021 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 1.875% |  | 1.875% |  |  |
Unsecured long-term debt, noncurrent | $ 146,000,000 |  | $ 146,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 1,383,000 |  | $ 1,807,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 6.25% due January 2038 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 6.25% |  | 6.25% |  |  |
Unsecured long-term debt, noncurrent | $ 36,000,000 |  | $ 36,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 228,000 |  | $ 239,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 4.75% due June 2040, mandatory tender - June 1, 2017 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage |  |  | 4.75% |  |  |
Unsecured long-term debt, noncurrent | 0 |  | $ 37,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 0 |  | 25,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.125% due June 2040, mandatory tender - June 1, 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 2.125% |  |  |  |  |
Unsecured long-term debt, noncurrent | $ 37,000,000 |  | 0 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 404,000 |  | $ 0 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.20% due June 2040, mandatory tender - June 1, 2020 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 5.20% |  | 5.20% |  |  |
Unsecured long-term debt, noncurrent | $ 40,045,000 |  | $ 40,045,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 105,000 |  | $ 147,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.90% due June 2040 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 5.90% |  | 5.90% |  |  |
Unsecured long-term debt, noncurrent | $ 255,000,000 |  | $ 255,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 2,040,000 |  | $ 2,131,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 6.25% due June 2040 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 6.25% |  | 6.25% |  |  |
Unsecured long-term debt, noncurrent | $ 11,500,000 |  | $ 11,500,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 92,000 |  | $ 96,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.54% due September 2042, mandatory tender - June 1, 2017 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage |  |  | 2.54% |  |  |
Unsecured long-term debt, noncurrent | 0 |  | $ 20,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 0 |  | 67,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.45% due September 2042, mandatory tender - June 1, 2022 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 2.45% |  |  |  |  |
Unsecured long-term debt, noncurrent | $ 20,000,000 |  | 0 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 153,000 |  | $ 0 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 2.40% due June 2043, mandatory tender - June 1, 2020 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 2.40% |  | 2.40% |  |  |
Unsecured long-term debt, noncurrent | $ 39,300,000 |  | $ 39,300,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 243,000 |  | $ 340,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.20% due June 2043, mandatory tender - June 1, 2020 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 5.20% |  | 5.20% |  |  |
Unsecured long-term debt, noncurrent | $ 21,000,000 |  | $ 21,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 53,000 |  | $ 75,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 7.95% due May 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 7.95% |  | 7.95% |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 7.50% due August 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 7.50% |  | 7.50% |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 5.35% due October 2021 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 5.35% |  | 5.35% |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes, Pollution Control Revenue Bonds: | 3.85% due August 2025 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 3.85% |  | 3.85% |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | 7.95% due May 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 7.95% |  |  |  |  |
Unsecured long-term debt, noncurrent | $ 350,000,000 | $ 450,000,000.0 | $ 350,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 272,000 |  | 995,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | 7.50% due August 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 7.50% |  |  |  |  |
Unsecured long-term debt, noncurrent | $ 100,025,000 |  | 100,025,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 73,000 |  | 197,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | 5.35% due October 2021 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Unsecured long-term debt, noncurrent | 160,000,000 |  | 160,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 617,000 |  | 780,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | 3.85% due August 2025 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage |  |  |  |  |  | 3.85%
Unsecured long-term debt, noncurrent | 250,000,000 |  | 250,000,000 |  |  | $ 250,000,000.0
Unamortized Discounts, (Premiums), and Issuance Costs, net | 2,274,000 |  | 2,574,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | PNM 2016 Term Loan Agreement due November 2017 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt | 0 |  | 175,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 0 |  | 28,000 |  |  |
Public Service Company of New Mexico | Senior Unsecured Notes: | PNM 2017 Term Loan Agreement due January 2019 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt | 200,000,000 |  | 0 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 23,000 |  | 0 |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total | 465,500,000 |  | 405,500,000 |  |  |
Principal, less current maturities | 0 |  | 0 |  |  |
Long-term debt, excluding current maturities, gross | 465,500,000 |  | 405,500,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | (15,120,000) |  | (15,375,000) |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities | 0 |  | 0 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities | $ (15,120,000) |  | $ (15,375,000) |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds: | 9.50% due April 2019 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 9.50% |  | 9.50% |  |  |
Unsecured long-term debt, noncurrent | $ 172,302,000 |  | $ 172,302,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 1,032,000 |  | $ 1,857,000 |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds: | 6.95% due April 2043 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 6.95% |  | 6.95% |  |  |
Unsecured long-term debt, noncurrent | $ 93,198,000 |  | $ 93,198,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ (18,057,000) |  | $ (18,773,000) |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds: | 4.03% due July 2024 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 4.03% |  | 4.03% |  |  |
Unsecured long-term debt, noncurrent | $ 80,000,000 |  | $ 80,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 686,000 |  | $ 792,000 |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds: | 3.53% due February 2026 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 3.53% |  | 3.53% |  | 3.53% |
Unsecured long-term debt, noncurrent | $ 60,000,000 |  | $ 60,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 667,000 |  | 749,000 |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds: | 3.22% due August 2027 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Stated percentage | 3.22% |  |  |  |  |
Unsecured long-term debt, noncurrent | $ 60,000,000 |  | 0 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 552,000 |  | 0 |  |  |
PNMR |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Total | 300,137,000 |  | 342,207,000 |  |  |
Principal, less current maturities | 257,268,000 |  | 42,025,000 |  |  |
Long-term debt, excluding current maturities, gross | 42,869,000 |  | 300,182,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 1,022,000 |  | 1,739,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, Less current maturities | 396,000 |  | 557,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net, excluding current maturities | 626,000 |  | 1,182,000 |  |  |
PNMR | PNMR 2015 Term Loan Agreement due March 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt | 150,000,000 |  | 150,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 12,000 |  | 84,000 |  |  |
PNMR | BTMU Term Loan Agreement, payments through February 2021 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt | 50,137,000 |  | 92,207,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | 1,001,000 |  | 1,634,000 |  |  |
PNMR | PNMR 2016 Two-Year Term Loan due December 2018 |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |
Long-term debt | 100,000,000 |  | 100,000,000 |  |  |
Unamortized Discounts, (Premiums), and Issuance Costs, net | $ 9,000 |  | $ 21,000 |  |  |

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