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Commitments and Contingencies - Coal Supply (Details) | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Aug. 01, 2016 payment | Jan. 31, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Other current assets | $ 53,863,000 | $ 54,808,000 |  |  |
PNM |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Other current assets | 43,650,000 | 43,516,000 |  |  |
PNM | Surface |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Regulatory assets | 100,000,000 |  |  |  |
PNM | Loss on long-term purchase commitment | Surface |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Loss contingency accrual | 70,400,000 | 70,100,000 |  |  |
Final reclamation, capped amount to be collected | 100,000,000.0 | 100,000,000 |  |  |
PNM | Loss on long-term purchase commitment | Underground |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Loss contingency accrual | 23,800,000 | 23,200,000 |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Estimate of possible loss | 102,600,000 |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Estimate of possible loss | 40,000,000 |  |  |  |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Other current assets | 26,300,000 | 26,300,000 |  |  |
Coal supply | NM Capital | San Juan Generating Station |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Requirement to post reclamation bonds | 118,700,000 |  |  |  |
Cash used to support bank letter or credit arrangement | 30,300,000 |  |  |  | $ 30,300,000
Increase in coal mine decommissioning liability | PNM | Underground and Surface |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Regulatory disallowance, additional amount recorded |  | 39,200,000 |  |  |
Increase in coal mine decommissioning liability | PNM | Surface |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Regulatory disallowance, additional amount recorded |  | 29,800,000 |  |  |
Increase in coal mine decommissioning liability | PNM | Underground |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Regulatory disallowance, additional amount recorded |  | 9,400,000 |  |  |
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 |  |
Mine Reclamation Trust | PNM |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year | 6,500,000 |  |  |  |
Reclamation trust funding, year 2 | 10,200,000 |  |  |  |
Reclamation trust funding, year 3 | 10,900,000 |  |  |  |
San Juan Generating Station | Loss on long-term purchase commitment | PNM |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |
Annual funding post-term reclamation trust |  | 10,000,000 |  |  |
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 |  |  |  |
Reclamation trust funding, year 3 | $ 2,300,000 |  |  |  |
Number of annual installment payments | payment |  |  |  | 13 |

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