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Financing - Short-term Debt and Liquidity (Details) | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2018 USD ($) derivative | Dec. 31, 2017 USD ($) | Nov. 01, 2020 USD ($) | Apr. 23, 2018 USD ($) | Apr. 09, 2018 USD ($) | Mar. 09, 2018 USD ($) | Feb. 26, 2018 USD ($) | Oct. 20, 2017 USD ($) | Jul. 28, 2017 USD ($) | Jul. 20, 2017 USD ($) | Dec. 21, 2016 USD ($) | Nov. 30, 2016 USD ($)
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt | $ 238,700,000 | $ 305,400,000 |  |  |  |  |  |  |  |  |  |
Letters of credit outstanding | 6,400,000 |  |  |  |  |  |  |  |  |  |  |
Maturities and other repayments due in next 12 months | 10,000,000 |  |  |  |  |  |  |  |  |  |  |
Maturities and other repayments due in the remainder of year two | 9,400,000 |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  | $ 300,000,000 |  |  |  |  |  |
Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  | $ 669,400,000 |  |  |  |  |  |  |  |
PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  |  |  |  |  |  | $ 0 |  |  |  |
PNMR | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  | 197,400,000 |  |  |  |  |  |  |  |
Consolidated invested cash |  |  |  | 900,000 |  |  |  |  |  |  |  |
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 | 2.63% |  |  |  |  |  |  |  |  |  |  |
Term of loan | 1 year |  |  |  |  |  |  |  |  |  |  |
Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Term of derivatives | 4 years |  |  |  |  |  |  |  |  |  |  |
Debt issued | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |
PNMR Development Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt | $ 21,500,000 | 0 |  |  |  |  |  |  |  |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  |  |  |  |  |  |  |  |  | $ 100,000,000.0 |
Term of loan | 2 years |  |  |  |  |  |  |  |  |  |  |
PNMR Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  |  |  |  |  |  |  |  |  |  | $ 10,000,000.0
PNMR Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt | 3.05% |  |  |  |  |  |  |  |  |  |  |
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 | 2.54% |  |  |  |  |  |  |  |  |  |  |
PNMR Development Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt | 2.72% |  |  |  |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt | $ 0 | 39,800,000 |  |  |  |  |  |  |  |  |  |
Letters of credit outstanding | 2,500,000 |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate | 54,600,000 | 0 |  |  |  |  |  |  |  |  |  |
PNM | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  | 397,500,000 |  |  |  |  |  |  |  |
Consolidated invested cash |  |  |  | 8,600,000 |  |  |  |  |  |  |  |
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 | Senior Unsecured Note Agreement (SUNs) | Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  |  | $ 450,000,000.0 |  |  |
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 | 21,200,000 | 0 |  |  |  |  |  |  |  |  |  |
Letters of credit outstanding | 100,000 |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate | 2,600,000 | 0 |  |  |  |  |  |  |  |  |  |
TNMP | First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt issued | 172,300,000 |  |  |  |  |  |  |  |  |  |  |
TNMP | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  | 34,500,000 |  |  |  |  |  |  |  |
Consolidated invested cash |  |  |  | 0 |  |  |  |  |  |  |  |
PNMR Development | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  |  |  | $ 68,200,000 |  |  |  |  |  |  |
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 | 96,000,000 | 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 | 21,200,000 | 0 |  |  |  |  |  |  |  |  |  |
Revolving credit facility | TNMP | First mortgage bonds |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Collateral amount | $ 75,000,000 |  |  |  |  |  |  |  |  |  |  |
Affiliated entity | TNMP | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  |  | $ 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) | $ 3,100,000 | $ 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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