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Commitments and Contingencies - Coal Supply (Details) |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 05, 2018 T | Jun. 29, 2018 USD ($) | Feb. 01, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | May 22, 2018 USD ($) | Feb. 01, 2018 | Jun. 30, 2017 USD ($) | Feb. 01, 2017 | Sep. 30, 2016 USD ($) | Aug. 01, 2016 payment | Jan. 31, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  | $ 53,068,000 |  | $ 53,068,000 |  | $ 47,358,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | (1,645,000) | $ 0 | 149,000 | $ 0 |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  | 46,467,000 |  | 46,467,000 |  | 39,904,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | (1,645,000) | $ 0 | 149,000 | $ 0 |  |  |  |  |  |  |  |
PNM | Surface |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | 100,000,000 |  | 100,000,000 |  |  |  |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | Surface |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) for interim storage costs liability |  |  |  |  |  |  | 30,000,000 |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  |  |  | 39,500,000 |  | 39,500,000 |  | 41,400,000 |  |  |  |  |  |  |
Final reclamation, capped amount to be collected |  |  |  |  |  |  | 100,000,000.0 |  |  |  |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | Underground |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) for interim storage costs liability |  |  |  |  |  |  | 10,000,000 |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  |  |  | 14,300,000 |  | 14,300,000 |  | 14,700,000 |  |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  | $ 16,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | 87,500,000 |  | 87,500,000 |  |  |  |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | $ 101,200,000 |  | $ 101,200,000 |  |  |  |  |  |  |  |  |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  |  |  |  |  | 26,300,000 |  |  |  |  |  |  |
Estimated increase in coal cost |  |  |  |  | 6.90% |  | 6.90% |  |  |  |  | 51.00% |  |  |  |
Coal supply | NM Capital | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  |  | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Loan agreement among several entities |  |  | 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  |  |  | 12.25% |  | 9.25% |  |  |
Requirement to post reclamation bonds |  |  |  |  | $ 118,700,000 |  | $ 118,700,000 |  |  |  |  |  |  |  |  |
Cash used to support bank letter or credit arrangement |  |  |  |  | 30,300,000 |  | 30,300,000 |  |  |  |  |  |  |  |  | $ 30,300,000
Coal supply | NM Capital | San Juan Coal Company, Westmoreland |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  |  | 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment penalty |  |  |  |  | 0 |  | 0 |  |  |  |  |  |  |  |  |
Repayment of line of credit |  |  |  |  |  |  |  |  |  | $ 50,100,000 |  |  |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  | 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of line of credit |  |  | $ 125,000,000.0 |  |  |  |  |  |  | $ 43,000,000 |  |  |  |  |  |
Increase in coal mine decommissioning liability | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,500,000 |  |
Increase in coal mine decommissioning liability | PNM | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  |  |  |  | 2,500,000 |  | 2,500,000 |  |  |  |  |  |  |  |  |
Mine Reclamation Trust | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  |  |  | 6,400,000 |  | 6,400,000 |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  |  |  | 8,700,000 |  | 8,700,000 |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  |  | 9,200,000 |  | 9,200,000 |  |  |  |  |  |  |  |  |
San Juan Generating Station | Loss on long-term purchase commitment | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual funding post-term reclamation trust |  |  |  |  |  |  |  |  | $ 5,800,000 |  |  |  |  |  |  |
Four Corners | Mine Reclamation Trust | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  |  |  |  |  |  |  |  |  |  | $ 2,300,000 |  |  |  |
Reclamation trust funding, year 2 |  |  |  |  | 2,300,000 |  | 2,300,000 |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  |  | $ 2,300,000 |  | $ 2,300,000 |  |  |  |  |  |  |  |  |
Number of annual installment payments | payment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13 |
Four Corners Coal Supply Arbitration | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for legal settlements |  | $ 4,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Four Corners Coal Supply Arbitration, Period March 1, 2018 through June 30, 2018 | PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for legal settlements |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event | Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in coal obligation | T | 111,668 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Continuous highwall mining | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNM's share estimated underpaid surface mining royalties under proposed rate change |  |  |  |  | 46.30% |  | 46.30% |  |  |  |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  |  |  |  | $ 5,000,000 |  | $ 5,000,000 |  |  |  |  |  |  |  |  |

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