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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) | 1 Months Ended | 4 Months Ended | 9 Months Ended | 13 Months Ended |  |  |  |
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Sep. 30, 2020 USD ($) | Sep. 30, 2012 landowner | Apr. 30, 2010 city | Sep. 30, 2020 USD ($) | Sep. 30, 2020 USD ($) generating_unit | Jun. 30, 2020 USD ($) | 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% |  |  | 8.00% | 8.00% |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change | $ 5,000,000 |  |  | $ 5,000,000 | $ 5,000,000 |  |  |  |  |
PNM's share estimated underpaid surface mining royalties under proposed rate change | 46.30% |  |  | 46.30% | 46.30% |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Franchise fee agreement term |  |  |  |  | 15 years |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Number of cities to provide cooling water | city |  |  | 5 |  |  |  |  |  |  |
Term of agreement for cooling water |  |  | 40 years |  |  |  |  |  |  |
PNM | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage in nuclear reactor | 10.20% |  |  | 10.20% | 10.20% |  |  |  |  |
Number of units | generating_unit |  |  |  |  | 3 |  |  |  |  |
Maximum potential assessment per incident | $ 42,100,000 |  |  | $ 42,100,000 | $ 42,100,000 |  |  |  |  |
Annual payment limitation related to incident | 6,200,000 |  |  | 6,200,000 | 6,200,000 |  |  |  |  |
Aggregate amount of all risk insurance | 2,800,000,000 |  |  | 2,800,000,000 | 2,800,000,000 |  |  |  |  |
Maximum amount under Nuclear Electric Insurance Limited | 5,400,000 |  |  | 5,400,000 | 5,400,000 |  |  |  |  |
Asset retirement obligation, revision of estimate | 2,800,000 |  |  | 9,200,000 |  | $ 6,400,000 |  |  |  |
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Liability insurance coverage | 13,800,000,000 |  |  | 13,800,000,000 | 13,800,000,000 |  |  |  |  |
Liability insurance coverage sublimit | 2,250,000,000 |  |  | 2,250,000,000 | 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 |  |  | 450,000,000 | 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 | $ 13,300,000,000 |  |  | $ 13,300,000,000 | $ 13,300,000,000 |  |  |  |  |
Pending litigation |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Written notification to terminate agreement, minimum period of time required |  |  |  |  | 30 days |  |  |  |  |

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