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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) |  | 1 Months Ended | 9 Months Ended |  |  |  |  |  |
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Mar. 28, 2017 | Sep. 30, 2012 landowner | Apr. 30, 2010 city | Sep. 30, 2018 USD ($) generating_unit | Oct. 31, 2018 USD ($) | Dec. 01, 2015 Allotment_Parcel | Jul. 13, 2015 a | Jan. 22, 2015 Allotment_Parcel | Feb. 27, 2014 lawsuit | 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% |  |  |  |  |  |
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% |  |  |  |  |  |
Number of units | generating_unit |  |  |  | 3 |  |  |  |  |  |
Maximum potential assessment per incident |  |  |  | $ 38,900,000.0 |  |  |  |  |  |
Annual payment limitation related to incident |  |  |  | 5,800,000.0 |  |  |  |  |  |
Aggregate amount of all risk insurance |  |  |  | 2,750,000,000.00 |  |  |  |  |  |
Maximum amount under Nuclear Electric Insurance Limited |  |  |  | 5,400,000.0 |  |  |  |  |  |
PNM | Maximum | Palo Verde Nuclear Generating Station | Nuclear plant |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Liability insurance coverage |  |  |  | 13,200,000,000.0 |  |  |  |  |  |
Liability insurance coverage sublimit |  |  |  | 2,250,000,000.00 |  |  |  |  |  |
First Choice |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Additional tax due plus penalties and interest |  |  |  | 5,000,000 |  |  |  |  |  |
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 |  |  |  | $ 12,700,000,000.0 |  |  |  |  |  |
Pending litigation |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Number of pending claims | lawsuit |  |  |  |  |  |  |  |  | 2 |
Written notification to terminate agreement, minimum period of time required | 30 days |  |  |  |  |  |  |  |  |
Settled litigation |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Number of pending claims | lawsuit |  |  |  |  |  |  |  |  | 1 |
Subsequent event | State and Local Jurisdiction | First Choice |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Adjustment from settlement with taxing authority |  |  |  |  | $ 900,000 |  |  |  |  |

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