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Financing - Financing Activities (Details) |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 28, 2017 USD ($) | Jul. 20, 2017 USD ($) | Feb. 01, 2016 USD ($) | Sep. 30, 2017 USD ($) derivative | Sep. 30, 2017 USD ($) Extension derivative | Dec. 31, 2016 USD ($) | Jun. 14, 2017 USD ($) | Oct. 21, 2016 USD ($) | Sep. 30, 2015 | Mar. 09, 2015 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated principal payments |  |  |  | $ 265,700,000 | $ 265,700,000 |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  |  |  | 1.927% |
Interest rate contract |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Number of hedging agreements (in derivatives) | derivative |  |  |  | 3 | 3 |  |  |  |  |
PNMR 2016 One-Year Term Loan |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Term of loan |  |  |  |  | 1 year |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  |  | 2.09% | 2.09% |  |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate at period end |  |  |  | 2.14% | 2.14% |  |  |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  | $ 100,000,000.0 | $ 100,000,000.0 |  |  |  |  |
Variable interest rate |  |  |  | 2.19% | 2.19% |  |  |  |  |
Term of loan |  |  |  |  | 2 years |  |  |  |  |
Variable rate short-term debt |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 50,000,000.0 | $ 50,000,000.0 |  |  |  |  |
Term of hedging agreement |  |  |  |  | 4 years |  |  |  |  |
Variable rate short-term debt | Interest rate contract, 1 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.926% | 1.926% |  |  |  |  |
Variable rate short-term debt | Interest rate contract, 2 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.823% | 1.823% |  |  |  |  |
Variable rate short-term debt | Interest rate contract, 3 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.629% | 1.629% |  |  |  |  |
Cash flow hedge | Level 2 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fair value gain (loss) (less than) |  |  |  |  | $ 300,000 | $ (100,000) |  |  |  |
Cash flow hedge | Level 2 | Interest rate contract, 3 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fair value gain (loss) (less than) |  |  |  |  | 100,000 |  |  |  |  |
Cash flow hedge | Level 2 | Interest rate contracts, 1 and 2 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Fair value gain (loss) (less than) |  |  |  |  | $ (500,000) |  |  |  |  |
PNM | Maximum |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Maturity term over which financings require regulator approval (more than) |  |  |  |  | 18 months |  |  |  |  |
NM Capital | Coal supply | San Juan Coal Company, Westmoreland |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  |  | $ 125,000,000.0 |  |  |  |  |  |  |
Line of credit |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  |  |  | 65.00% |  |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  | $ 150,000,000.0
PNM 2016 Term Loan Agreement | PNM | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | 175,000,000.0 |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Term loans |  |  | $ 125,000,000.0 | $ 60,900,000 | $ 60,900,000 |  |  |  |  |
Estimated principal payments |  |  |  | $ 15,700,000 | $ 15,700,000 |  |  |  |  |
Letter of credit | PNMR | JPM LOC Facility |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 30,300,000.0 |  |
First mortgage bonds | TNMP | Interest Rate of 3.22%, Due 2027 | Plan |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  | 3.22% |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 60,000,000.0 |  |  |
Pollution Control Revenue Bonds | PNM | Pollution Control Revenue Bonds due June 1, 2040 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 2.125% | 2.125% |  |  |  |  |
Outstanding balance |  |  |  | $ 37,000,000 | $ 37,000,000 | 37,000,000 |  |  |  |
Pollution Control Revenue Bonds | PNM | Pollution Control Revenue Bonds due June 1, 2042 |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 2.45% | 2.45% |  |  |  |  |
Outstanding balance |  |  |  | $ 20,000,000 | $ 20,000,000 | $ 20,000,000 |  |  |  |
Term loan agreement with banks | PNM | PNM 2017 Term Loan Agreement | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Covenant that requires the maintenance of a debt-to-capital ratio (less than or equal to) |  | 65.00% |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 200,000,000.0 |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | Senior Unsecured Note Agreement (SUNs) |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Covenant that requires the maintenance of a debt-to-capital ratio (less than or equal to) | 65.00% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 450,000,000.0 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018 | Plan |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount | 350,000,000.0 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018 | Plan |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 100,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.15% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 3.15% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 55,000,000.0 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.45% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 3.45% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 104,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.68% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 3.68% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 88,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.93% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 3.93% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 38,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.22% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 4.22% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 45,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.50% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 4.50% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 20,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 3.78% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 3.78% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 15,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 4.60% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate | 4.60% |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 85,000,000 |  |  |  |  |  |  |  |  |
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in May 2018, 7.95% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 7.95% | 7.95% |  |  |  |  |
Amount of senior unsecured notes to be repaid with SUN proceeds |  |  |  | $ 350,000,000 |  |  |  |  |  |
Senior Unsecured Notes |  |  |  | $ 450,000,000 | $ 450,000,000 |  |  |  |  |
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in August 2018, 7.50% |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 7.50% | 7.50% |  |  |  |  |
Senior Unsecured Notes |  |  |  | $ 100,000,000.0 | $ 100,000,000.0 |  |  |  |  |
PNM 2017 Term Loan Agreement |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate at period end |  |  |  | 1.97% | 1.97% |  |  |  |  |
PNMR Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  |  | 2.49% | 2.49% |  |  |  |  |
LIBOR | BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 4.06% | 4.06% |  |  |  |  |
Revolving credit facility | TNMP |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Number of extension options | Extension |  |  |  |  | 2 |  |  |  |  |
Extension option on term |  |  |  |  | 1 year |  |  |  |  |

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