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Commitments and Contingencies - Coal Supply (Details) |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) | Oct. 20, 2017 USD ($) | Jun. 30, 2017 | Feb. 01, 2017 | Aug. 01, 2016 payment
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 56,421,000 |  | $ 56,421,000 |  | $ 73,444,000 |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  | 0 | $ 16,451,000 | 0 | $ 17,225,000 |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | 48,883,000 |  | 48,883,000 |  | 67,355,000 |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  | 0 | 16,451,000 | 0 | $ 17,225,000 |  |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  |  | 41,400,000 |  | 41,400,000 |  | 41,000,000 |  |  |  |
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] |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  |  | 14,700,000 |  | 14,700,000 |  | 14,000,000 |  |  |  |
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 |  |  |  | 100,900,000 |  | 100,900,000 |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  | 127,400,000 |  | 127,400,000 |  |  |  |  |  |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | 31,900,000 |  | 31,900,000 |  | 48,700,000 |  |  |  |
Estimated increase in coal cost |  |  |  |  |  |  |  |  |  | 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 |  | 7.25% |  |  |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  |  |  |
Loan receivable |  | $ 125,000,000 |  | 66,200,000 |  | 66,200,000 |  |  |  |  |  |
Scheduled principal payment proceeds |  |  |  | 9,600,000 |  | 9,600,000 |  |  |  |  |  |
Scheduled interest payment proceeds |  |  |  | 1,800,000 |  | 1,800,000 |  |  |  |  |  |
Prepayment penalty |  |  |  | 0 |  | 0 |  |  |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long-term Debt | $ 125,000,000.0 |  |  | 60,900,000 |  | 60,900,000 |  |  |  |  |  |
Subsequent event | Coal supply | Unaffiliated third party | San Juan Coal Company, Westmoreland |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount held in a restricted bank account |  |  |  |  |  |  |  |  | $ 11,400,000 |  |  |
Increase in coal mine decommissioning liability | PNM |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Expense reflected in regulatory disallowances and restructuring costs |  |  |  |  | $ 4,800,000 |  |  | $ 4,500,000 |  |  |  |
Mine Reclamation Trust | PNM |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  |  | 5,800,000 |  | 5,800,000 |  |  |  |  |  |
Reclamation Trust Funding, Year 2 |  |  |  | 8,300,000 |  | 8,300,000 |  |  |  |  |  |
Reclamation Trust Funding, Year 3 |  |  |  | 8,700,000 |  | 8,700,000 |  |  |  |  |  |
Four Corners | Mine Reclamation Trust | PNM |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  |  | 2,300,000 |  | 2,300,000 |  |  |  |  |  |
Reclamation Trust Funding, Year 2 |  |  |  | 2,100,000 |  | 2,100,000 |  |  |  |  |  |
Reclamation Trust Funding, Year 3 |  |  |  | $ 2,100,000 |  | $ 2,100,000 |  |  |  |  |  |
Number of annual installment payments | payment |  |  |  |  |  |  |  |  |  |  |  | 13

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