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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) | 1 Months Ended | 6 Months Ended |  |  |  |
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Sep. 30, 2012 landowner | Jun. 30, 2021 USD ($) lawsuit generating_unit plaintiff shareholder | Dec. 01, 2015 Allotment_Parcel | Jul. 13, 2015 a | Jan. 22, 2015 Allotment_Parcel | Aug. 31, 2013
Continuous Highwall Mining | San Juan Generating Station |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Proposed retroactive surface mining royalty rate |  |  |  |  |  | 12.50%
Surface mining royalty rate applied |  | 8.00% |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  | $ 5,000,000 |  |  |  |
PNM's share estimated underpaid surface mining royalties under proposed rate change |  | 46.30% |  |  |  |
Merger-Related Litigation |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Number of shareholders involved in lawsuit | shareholder |  | 6 |  |  |  |
Number of lawsuits for the Southern District of New York | lawsuit |  | 5 |  |  |  |
Number of lawsuits filed for the Eastern District of New York | lawsuit |  | 1 |  |  |  |
Number of plaintiffs who filed notices of voluntary dismissal | plaintiff |  | 5 |  |  |  |
PNM | Navajo Nation Allottee Matters |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Number of landowners involved in the appeal | landowner | 43 |  |  |  |  |
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel |  |  | 2 |  | 10 |
Area of land (in acres) | a |  |  |  | 15.49 |  |
Number of allotment parcels at issue that are not to be condemned | Allotment_Parcel |  |  | 2 |  |  |
Number of allotment parcels at issue | Allotment_Parcel |  |  | 5 |  |  |
PNM | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Ownership percentage in nuclear reactor |  | 10.20% |  |  |  |
Number of units | generating_unit |  | 3 |  |  |  |
Maximum potential assessment per incident |  | $ 42,100,000 |  |  |  |
Annual payment limitation related to incident |  | 6,200,000 |  |  |  |
Aggregate amount of all risk insurance |  | 2,800,000,000 |  |  |  |
Maximum amount under Nuclear Electric Insurance Limited |  | 5,400,000 |  |  |  |
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Liability insurance coverage |  | 13,500,000,000 |  |  |  |
Liability insurance coverage sublimit |  | 2,250,000,000 |  |  |  |
Commercial providers | PNM | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Liability insurance coverage |  | 450,000,000 |  |  |  |
Industry Wide Retrospective Assessment Program | PNM | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Liability insurance coverage |  | $ 13,100,000,000 |  |  |  |

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