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Stockholders' Equity (Details) - USD ($) |  | 12 Months Ended |
--- | --- | --- | ---
Dec. 15, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 07, 2020
Class of Stock [Line Items] |  |  |  |  |
Payment of dividends |  | $ 423,700,000 |  |  |
PNMR 2020 Forward Equity Sales Agreement |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Number of shares issued in transaction (in shares) | 6,200,000 |  |  |  |
Sale of stock, price per share (in dollars per share) | $ 45.805 |  |  |  | $ 47.21
Sale of stock, consideration received on transaction | $ 283,100,000 |  |  |  |
Payments of stock issuance costs | $ 100,000 |  |  |  |
Line of Credit |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Debt-to-capital ratio (not more than) |  | 70.00% |  |  |
Public Service Company of New Mexico |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Equity contribution from parent |  | $ 230,000,000 | $ 0 | $ 0 |
Preferred stock, dividend rate |  | 4.58% |  |  |
Preferred stock, redemption percent |  | 102.00% |  |  |
Preferred stock outstanding (in shares) |  | 115,293 | 115,293 |  |
Preferred stock, cumulative shares authorized (in shares) |  | 10,000,000 | 10,000,000 |  |
Public Service Company of New Mexico | Maximum |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Requirement to obtain approval to transfer more than a percentage of PNM's assets |  | 5.00% |  |  |
Public Service Company of New Mexico | Line of Credit |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Debt-to-capital ratio (not more than) |  | 65.00% |  |  |
Public Service Company of New Mexico | Affiliated Entity |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Cash dividends paid to parent company by consolidated subsidiaries |  | $ 40,700,000 | $ 0 | 77,400,000 |
Texas-New Mexico Power Company |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Equity contribution from parent |  | $ 71,000,000 | 80,000,000 | 30,000,000 |
Preferred stock, cumulative shares authorized (in shares) |  | 1,000,000 |  |  |
Texas-New Mexico Power Company | Affiliated Entity |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Cash dividends paid to parent company by consolidated subsidiaries |  | $ 58,500,000 | $ 55,300,000 | $ 41,900,000 |
PNMR and TNMP |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Preferred stock outstanding (in shares) |  | 0 |  |  |

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