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Financing - Financing Activities (Details) |  |  |  |  |  |  |  |  |  | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 22, 2020 USD ($) | Apr. 30, 2020 USD ($) | Apr. 24, 2020 USD ($) bond_series | Apr. 15, 2020 USD ($) | Apr. 01, 2020 USD ($) | Jan. 08, 2020 $ / shares shares | Jan. 07, 2020 shares | Jan. 01, 2020 | Jul. 01, 2019 USD ($) | Jun. 30, 2020 USD ($) shares | Jun. 30, 2019 | Jul. 24, 2020 | Jul. 15, 2020 USD ($) | Jun. 01, 2020 USD ($) | Dec. 31, 2019 USD ($) shares | Oct. 21, 2016 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  |  |  |  |  |  |  |  | 79,653,624 |  |  |  |  | 79,653,624 |
PNMR 2020 Forward Equity Sales Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  |  |  |  |  | 6,200,000 |  |  |  |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | $ / shares |  |  |  |  |  | $ 47.21 |  |  |  |  |  |  |  |  |  |
Over-Allotment Option |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued in transaction (in shares) | shares |  |  |  |  |  | 800,000 | 800,000 |  |  |  |  |  |  |  |  |
PNMR 2018 Two Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 0.98% |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |
Term of loan |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |  |  |
PNMR 2019 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 1.14% |  |  |  |  |  |
PNMR Development Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 90,000,000.0 |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  |  |  |  |  |  |  |  | 39,117,799 |  |  |  |  | 39,117,799 |
PNM | PNM 2019 40 Million Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 40,000,000.0 |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 0.84% |  |  |  |  |  |
PNM | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity term over which financings require regulator approval (more than) |  |  |  |  |  |  |  |  |  | 18 months |  |  |  |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) | shares |  |  |  |  |  |  |  |  |  | 6,358 |  |  |  |  | 6,358 |
Line of credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  |  |  |  |  |  |  |  | 65.00% | 70.00% |  |  |  |  |
Line of credit | PNM | PNM Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Letter of credit | PNMR | JPM LOC Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,300,000
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 185,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Series of bond issuance | bond_series |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 185,000,000.0 |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 1 and 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 110,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 85,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  | 2.73% |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 25,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  | 3.36% |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 3 and 4 | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | 75,000,000.0 |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 3 | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,000,000.0 |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 2.93% |  |  |
Mortgage Bonds | Texas-New Mexico Power Company | TNMP 2020 Bond Purchase Agreement, Bond Series 4 | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 50,000,000.0 |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 3.36% |  |  |
Medium-term Notes | PNM | PNM 2020 250 Million Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 250,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 2.70% |  |  |  |  |  |
Medium-term Notes | PNM | PNM 2019 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  | $ 250,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |  |  |
Senior Notes | Senior Unsecured Notes, 3.21% Series A, due April 2030 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 150,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  | 3.21% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | Senior Unsecured Notes, 3.57% Series B, due April 2039 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  | 3.57% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 200,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Notes | PNM | PNM 2020 250 Million Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of debt |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bonds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | 302,500,000 |  |  |  |  |  |
Repayments of debt | $ 302,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bonds | City of Farmington, New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 266,500,000 |  |
Bonds | Pollution Control Revenue Bonds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | 100,300,000 |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,300,000 |
Bonds | Pollution Control Revenue Bonds | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  | 0.47% |  |  |  |
Bonds | Pollution Control Revenue Bonds, Due 2040 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40,000,000.0 |
Bonds | Pollution Control Revenue Bonds, Due 2043, Series 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 39,300,000 |
Bonds | Pollution Control Revenue Bonds, Due 2043, Series 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21,000,000.0 |
Bonds | Pollution Control Revenue Bonds, 5.90%, Callable | City of Farmington, New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 255,000,000.0 |  |  |  |  | $ 255,000,000.0 |
Stated interest rate |  |  |  |  |  |  |  |  |  | 5.90% |  |  |  |  | 5.90% |
Bonds | Pollution Control Revenue Bonds, 6.25%, Callable | Maricopa County, Arizona Pollution Control Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 36,000,000.0 |  |  |  |  | $ 36,000,000.0 |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.25% |
Debt instrument, redemption price, percentage |  |  |  |  |  |  |  | 101.00% |  |  |  |  |  |  |  |
Bonds | Pollution Control Revenue Bonds, 6.25%, Callable | City of Farmington, New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 11,500,000 |  |  |  |  | $ 11,500,000 |
Stated interest rate |  |  |  |  |  |  |  |  |  | 6.25% |  |  |  |  | 6.25% |
Bonds | Pollution Control Revenue Bonds, 1.05 Percent Due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 36,000,000.0 |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  | 1.05% |  |  |  |  |  |
Bonds | Pollution Control Revenue Bonds, 1.20 Percent Due 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 11,500,000 |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  | 1.20% |  |  |  |  |  |
Bonds | Pollution Control Revenue Bonds, 1.10 Percent Due 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 130,000,000.0 |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  | 1.10% |  |  |  |  |  |
Bonds | Pollution Control Revenue Bonds, 1.15 Percent Due 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 125,000,000.0 |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  | 1.15% |  |  |  |  |  |
Term loan agreement with banks | PNM | PNM 2020 250 Million Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 150,000,000.0 |  |  |  |  |  |
Term loan agreement with banks | PNM | PNM 2019 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 150,000,000.0 |  |  |  |  |  |
Term loan agreement with banks | PNM | PNM 2020 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 2.70% |  |  |  |  |  |
Term loan agreement with banks | Texas-New Mexico Power Company | PNMR Development Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  | $ 90,000,000.0 |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  | 0.98% |  |  |  |  |  |
San Juan Generating Station | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Request issuance of energy transition bonds |  |  |  |  | $ 361,000,000 |  |  |  | $ 361,000,000 |  |  |  |  |  |  |

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- Definition Debt Instrument, Covenant, Debt To Capitalization Ratio + References No definition available. + Details
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- Definition Debt Instrument, Series Of Bond Issuance + References No definition available. + Details
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- Definition Public Utilities, Maturity term over which financings require regulator approval + References No definition available. + Details
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- Definition Public Utilities, Request Issuance Of Energy Transition Bonds + References No definition available. + Details
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Namespace Prefix: | pnm_
Data Type: | xbrli:monetaryItemType
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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