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Financing - Activities, PNM and TNMP (Details) |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2018 USD ($) | Jul. 28, 2017 USD ($) | Feb. 01, 2016 USD ($) | Dec. 21, 2015 USD ($) | Aug. 11, 2015 USD ($) | May 08, 2015 USD ($) | Jan. 01, 2015 USD ($) | Jun. 21, 2016 USD ($) | Dec. 31, 2017 USD ($) derivative | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Feb. 20, 2018 USD ($) | Sep. 30, 2017 USD ($) | Jul. 20, 2017 USD ($) | Jun. 14, 2017 USD ($) | Dec. 21, 2016 USD ($) loan | Oct. 21, 2016 USD ($) | Sep. 27, 2016 USD ($) | May 20, 2016 USD ($) | Dec. 17, 2015 USD ($) | Sep. 30, 2015 | Jun. 01, 2015 USD ($) | Mar. 09, 2015 USD ($) | Jan. 01, 2014 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding amount |  |  |  |  |  |  |  |  | $ 305,400,000 | $ 287,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  |  |  |  |  |  |  |  | 100,000,000 | $ 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of debt |  |  |  |  |  |  |  |  | 274,070,000 | 303,793,000 | 333,066,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.927% |  |  |
PNMR 2016 One-Year Term Loan and PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of loan agreements (in loans) | loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |
PNMR 2016 One Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding amount |  |  |  |  |  |  |  |  | $ 100,000,000 | 100,000,000 |  |  |  |  |  | $ 100,000,000.0 |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  | 2.32% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNMR 2016 Two-Year Term Loan due December 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100,000,000.0 |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  | 2.32% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Time hedged in interest rate cash flow hedge |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Level 2 | Cash Flow Hedging |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value gain (loss, less than) |  |  |  |  |  |  |  |  | $ 200,000 | (100,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Notes Payable to Banks | PNMR Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000.0
Amount outstanding |  |  |  | $ 150,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NM Capital | San Juan Generating Station | Coal Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  |  | $ 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNMR | PNMR 2016 Two-Year Term Loan due December 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  |  |  | 100,000,000 | 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding amount |  |  |  |  |  |  |  |  | 39,800,000 | 61,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of debt |  |  |  |  |  |  |  |  | $ 232,000,000 | 271,000,000 | $ 214,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requirement to obtain approval for any financing transaction, period of time (more than) |  |  |  |  |  |  |  |  | 18 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt-to-capital ratios (less than or equal to) |  |  |  |  |  |  |  |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit | Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt-to-capital ratios (less than or equal to) |  |  |  |  |  |  |  |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 150,000,000.0 |  |  |  |  |  |  | $ 150,000,000.0 |
Stated percentage |  |  |  |  |  |  |  |  | 2.34% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | Senior unsecured notes, 9.25% due 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 118,800,000
Stated percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9.25%
Unsecured Debt | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shelf registration statement, capacity of issuance (up to) |  |  |  |  |  |  |  |  | $ 475,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | Public Service Company of New Mexico | 3.85% due August 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  | $ 250,000,000.0 |  |  |  | 250,000,000 | 250,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  | 3.85% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  |  |  | 200,000,000 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | Public Service Company of New Mexico | 7.95% due May 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  | $ 350,000,000 | 350,000,000 |  |  | $ 450,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 7.95% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt |  | $ 350,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured Debt | Public Service Company of New Mexico | 7.50% due August 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  | $ 100,025,000 | 100,025,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 7.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  | $ 125,000,000.0 |  |  |  |  |  | $ 50,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 4.13% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated principal payments |  |  |  |  |  |  |  |  | $ 10,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
BTMU Term Loan Agreement | NM Capital | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  |  |  |  |  |  | $ 45,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Payment of debt | $ 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Excess amount of scheduled payment | 3,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Westmoreland loan | NM Capital | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment of debt | 5,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Excess amount of scheduled payment | $ 4,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Letter of Credit | PNMR | Letter or credit, 30 mil JP Morgan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,300,000.0 |  |  |  |  |  |  |
PNM 2014 Term Loan Agreement | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  | $ 175,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNM 2014 Multi-Draw Term Loan Agreement | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan entered into |  |  |  |  |  |  | $ 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings |  |  |  |  |  | $ 25,000,000 | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  |  |  |  | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unsecured notes, Pollution control revenue bonds | Public Service Company of New Mexico | Senior unsecured note, PCRB Due 2043, at 4 percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 39,300,000
Unsecured notes, Pollution control revenue bonds | Public Service Company of New Mexico | Senior Unsecured Note PCRB Dude 2020 at 2.45 Percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 39,300,000 |  |
Stated percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.45% |  |
PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 175,000,000.0 |  |  |  |  |
Senior Unsecured Notes | Public Service Company of New Mexico | Senior Unsecured Note Agreement (SUNs) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 450,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt to capital ratio |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 1 Point 875 Percent, due 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.875% |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 146,000,000.0 |  |  |  |  |  |
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.125 Percent, due 2040 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  | 2.125% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 37,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.45 Percent, due 2042 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  | 2.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 20,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
First Mortgage Bonds | Texas-New Mexico Power Company | 4.03% due 2024, Series 2014A |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  | $ 80,000,000 | $ 80,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 4.03% | 4.03% |  |  |  |  |  |  |  |  |  |  |  |  |  |
First Mortgage Bonds | Texas-New Mexico Power Company | 3.53% due February 2026 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate outstanding principal amount |  |  |  |  |  |  |  |  | $ 60,000,000 | $ 60,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 3.53% | 3.53% |  |  |  |  |  |  |  |  |  | 3.53% |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 60,000,000.0 |  |  |  |
JPMorgan Chase Bank, N.A. | PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 175,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
JPMorgan Chase Bank, N.A. | Notes Payable to Banks | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  | 2.29% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000.0 |  |  |  |  |  |  |  |  |  |
Scenario, plan | Senior Unsecured Notes | Public Service Company of New Mexico | SUNs, Issuance in May 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 350,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Scenario, plan | Senior Unsecured Notes | Public Service Company of New Mexico | SUN's, Issuance in August 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Scenario, plan | First Mortgage Bonds | Texas-New Mexico Power Company | Interest Rate of 3.22%, Due 2027 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.22% |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 60,000,000.0 |  |  |  |  |  |  |  |  |
Interest rate contract |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of derivative instruments held | derivative |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate contract one | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  | 1.926% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate contract two | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  | 1.823% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate contract three | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  | 1.629% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate contract three | Level 2 | Cash Flow Hedging |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value gain (loss, less than) |  |  |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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