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Financing - Short-term Debt and Liquidity (Details) |  | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
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Sep. 26, 2018 Extension | Jul. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) derivative | Nov. 01, 2020 USD ($) | Oct. 30, 2018 USD ($) | Jul. 25, 2018 | Jun. 28, 2018 USD ($) | Mar. 09, 2018 USD ($) | Feb. 26, 2018 USD ($) | Jul. 20, 2017 USD ($) | Dec. 21, 2016 USD ($)
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 262,600,000 | $ 305,400,000 |  |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 4,700,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |
Stated interest rate |  |  |  |  |  |  |  |  | 3.25% |  |  |
Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | $ 653,700,000 |  |  |  |  |  |
PNMR | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | 168,800,000 |  |  |  |  |  |
Consolidated invested cash |  |  |  |  |  | 900,000 |  |  |  |  |  |
PNMR Development Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 24,500,000 | 0 |  |  |  |  |  |  |  |
PNMR 2016 One-Year Term Loan (as extended) |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 100,000,000 | $ 100,000,000 |  |  |  |  |  |  |  | $ 100,000,000.0
Weighted-average interest rate for short-term debt |  |  | 3.02% |  |  |  |  |  |  |  |  |
Term of loan |  | 1 year | 1 year |  |  |  |  |  |  |  |  |
Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Term of derivatives |  |  |  | 4 years |  |  |  |  |  |  |  |
Debt issued |  |  |  | $ 50,000,000.0 |  |  |  |  |  |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000.0
Term of loan |  | 2 years | 2 years |  |  |  |  |  |  |  |  |
PNMR Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | $ 10,000,000.0 |  |  |  |  |  |  |  |  |
Number of extensions | Extension | 2 |  |  |  |  |  |  |  |  |  |  |
Lease term extension period | 1 year |  |  |  |  |  |  |  |  |  |  |
PNMR Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  | 3.34% |  |  |  |  |  |  |  |  |
PNMR Revolving Credit Facility | Forecast |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  |  |  | $ 290,000,000.0 |  |  |  |  |  |  |
TNMP Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  | 2.99% |  |  |  |  |  |  |  |  |
PNMR Development Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  | 3.07% |  |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 0 | 39,800,000 |  |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 2,500,000 |  |  |  |  |  |  |  |  |
PNM | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | 397,500,000 |  |  |  |  |  |
Consolidated invested cash |  |  |  |  |  | 55,300,000 |  |  |  |  |  |
PNM | Lines of credit |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
NMPRC approved credit facility |  |  | 40,000,000 |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 0 | 0 |  |  |  |  |  |  |  |
PNM | Lines of credit | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | 40,000,000 |  |  |  |  |  |
PNM | SUNs, Issuance in August 2018 | Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |
PNM | Senior Unsecured Notes, Due 2018, at 7 point 50 percent | Unsecured debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  | 7.50% |  |  |  |  |  |  |  |  |
PNM | Plan | SUNs, Issuance in August 2018 | Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  | $ 100,000,000 |  |  |  |  |  |  |  |  |
PNM | PNM Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 40,000,000.0 |  |  |  |  |  |  |  |  |
PNM | PNM Revolving Credit Facility | Forecast |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  |  |  | $ 360,000,000.0 |  |  |  |  |  |  |
TNMP |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 17,500,000 | 0 |  |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 100,000 |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  | 4,100,000 | 0 |  |  |  |  |  |  |  |
TNMP | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | 47,400,000 |  |  |  |  |  |
Consolidated invested cash |  |  |  |  |  | 0 |  |  |  |  |  |
TNMP | First Mortgage Bonds | Mortgages |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  | 172,300,000.0 |  |  |  |  | $ 60,000,000.0 |  |  |  |
Stated interest rate |  |  |  |  |  |  | 2.94% | 3.85% |  |  |  |
PNMR Development | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  | 0 |  |  |  |  |  |
PNMR Development | PNMR 2018 Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  |  |  |  |  |  |  |  | $ 24,500,000.0 |  |
Revolving credit facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 300,000,000.0 |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 120,600,000 | 165,600,000 |  |  |  |  |  |  |  |
Revolving credit facility | PNM |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 400,000,000.0 |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 0 | 39,800,000 |  |  |  |  |  |  |  |
Revolving credit facility | TNMP |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 75,000,000.0 |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 17,500,000 | $ 0 |  |  |  |  |  |  |  |
Revolving credit facility | TNMP | First mortgage bonds |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Collateral amount |  |  | 75,000,000 |  |  |  |  |  |  |  |  |
Affiliated entity | PNM | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  |  |  |  | 0 |  |  |  |  |  |
Affiliated entity | TNMP | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  |  |  |  | $ 5,400,000.0 |  |  |  |  |  |
Interest rate contract |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of derivatives | derivative |  |  |  | 3 |  |  |  |  |  |  |  |
Interest rate 1 | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.926% |  |  |  |  |  |  |  |
Interest rate 2 | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.823% |  |  |  |  |  |  |  |
Interest rate 3 | Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  | 1.629% |  |  |  |  |  |  |  |
Interest rate 3 | Level 2 | Cash Flow Hedge |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Fair value gain (loss) |  |  | $ 4,000,000 | $ 1,400,000 |  |  |  |  |  |  |  |
JPMorgan Chase Bank, N.A. and U.S. Bank National Association | PNM | PNM 2017 Term Loan Agreement | Term loan agreement with banks |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  |  |  |  | $ 200,000,000.0 |

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