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Financing - Short-term Debt and Liquidity (Details) |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 01, 2019 USD ($) | Jan. 18, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) derivative | Jul. 01, 2019 USD ($) | May 03, 2019 USD ($) | Mar. 29, 2019 USD ($) | Feb. 26, 2019 USD ($) | Feb. 22, 2019 USD ($) | Feb. 21, 2019 USD ($)
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  |  |  |  |  |  |  |  | $ 25,000,000.0 | $ 24,500,000.0
Short-term debt |  |  | $ 223,800,000 | $ 235,900,000 |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 4,700,000 |  |  |  |  |  |  |  |
Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | $ 732,500,000 |  |  |  |
PNMR | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | 239,400,000 |  |  |  |
Consolidated invested cash |  |  |  |  |  |  | 900,000 |  |  |  |
PNMR 2018 One-Year Term Loan |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 150,000,000 | 150,000,000 |  |  |  |  |  |  |
Term of loan |  |  | 1 year |  |  |  |  |  |  |  |
PNMR 2018 One-Year Term Loan | PNMR |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  | 3.24% |  |  |  |  |  |  |  |
PNMR Development Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 11,600,000 | 6,000,000 |  |  |  |  |  |  |
Variable Rate Short-Term Debt |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Term of derivatives |  |  |  |  | 4 years |  |  |  |  |  |
Debt issued |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |
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 |  |  | 3.74% |  |  |  |  |  |  |  |
PNMR Development Revolving Credit Facility | PNMR |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Weighted-average interest rate for short-term debt |  |  | 3.49% |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | $ 0 | 42,400,000 |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 2,500,000 |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  | 0 | 19,800,000 |  |  |  |  |  |  |
PNM | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | 397,500,000 |  |  |  |
Consolidated invested cash |  |  |  |  |  |  | 9,000,000 |  |  |  |
PNM | Lines of credit |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
NMPRC approved credit facility |  |  | 40,000,000 |  |  |  |  |  |  |  |
Short-term debt |  |  | 0 | 10,000,000 |  |  |  |  |  |  |
PNM | Lines of credit | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | 40,000,000 |  |  |  |
PNM | PNM Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 40,000,000.0 |  |  |  |  |  |  |  |
TNMP |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term debt |  |  | 0 | 17,500,000 |  |  |  |  |  |  |
Letters of credit outstanding |  |  | 700,000 |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  | 0 | 100,000 |  |  |  |  |  |  |
TNMP | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | 45,700,000 |  |  |  |
Consolidated invested cash |  |  |  |  |  |  | 0 |  |  |  |
TNMP | TNMP 2019 Bond Purchase Agreement |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  | $ 225,000,000 | $ 305,000,000.0 |  |
TNMP | First mortgage bonds |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt |  | $ 172,300,000 |  |  |  |  |  |  |  |  |
Stated interest rate |  | 9.50% |  |  |  |  |  |  |  |  |
TNMP | PNMR 2018 One-Year Term Loan |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  |  |  | 150,000,000 |  |  |
TNMP | First Mortgage Bonds 3.60 Percent Due 2029 | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  | $ 80,000,000 |  |  |  |  |
PNMR Development |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term debt – affiliate |  |  | 3,100,000 | 0 |  |  |  |  |  |  |
PNMR Development | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Remaining borrowing capacity |  |  |  |  |  |  | $ 9,900,000 |  |  |  |
Revolving credit facility |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 300,000,000.0 |  |  |  |  |  |  |  |
Short-term debt |  |  | 62,200,000 | 20,000,000 |  |  |  |  |  |  |
Revolving credit facility | PNM |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 400,000,000.0 |  |  |  |  |  |  |  |
Short-term debt |  |  | 0 | 32,400,000 |  |  |  |  |  |  |
Revolving credit facility | TNMP |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Financing capacity |  |  | 75,000,000.0 |  |  |  |  |  |  |  |
Short-term debt |  |  | 0 | 17,500,000 |  |  |  |  |  |  |
Revolving credit facility | TNMP | First mortgage bonds |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Collateral amount |  |  | 75,000,000 |  |  |  |  |  |  |  |
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) |  |  | 1,400,000 |  |  |  |  |  |  |  |
Fair value gain (loss) |  |  |  | $ 1,000,000 |  |  |  |  |  |  |
Mortgages | TNMP | TNMP 2019 Bond Purchase Agreement |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  | $ 305,000,000 |  |  |  |  | $ 225,000,000 | $ 305,000,000.0 |  |
Stated interest rate |  |  |  |  |  |  |  |  | 400.00% |  |
Mortgages | TNMP | First Mortgage Bonds 3.60 Percent Due 2029 | Subsequent event |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt issued |  |  |  |  |  | $ 80,000,000 |  |  |  |  |
Stated interest rate |  |  |  |  |  | 3.60% |  |  |  |  |
Unsecured Debt | TNMP | First mortgage bonds |  |  |  |  |  |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt | $ 172,300,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate | 9.50% |  |  |  |  |  |  |  |  |  |

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