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Commitments and Contingencies - Coal Supply (Details) |  |  |  | 6 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 29, 2018 USD ($) | Mar. 17, 2018 T | Feb. 01, 2016 USD ($) | Jun. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | May 22, 2018 USD ($) | Feb. 01, 2018 | Jun. 30, 2017 USD ($) | Feb. 01, 2017 | Aug. 01, 2016 payment | Jan. 31, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 51,818,000 | $ 47,358,000 |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | 46,369,000 | 39,904,000 |  |  |  |  |  |
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 |  |  |  | 40,500,000 | 41,400,000 |  |  |  |  |  |
Increase (decrease) for interim storage costs liability |  |  |  |  | 35,000,000 |  |  |  |  |  |
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 |  |  |  | 15,400,000 | 14,700,000 |  |  |  |  |  |
Increase (decrease) for interim storage costs liability |  |  |  |  | 5,000,000 |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  | 98,500,000 |  |  |  |  |  |  |
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  | 127,100,000 |  |  |  |  |  |  |
Coal supply | PNM | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 26,300,000 | 26,300,000 |  |  |  |  |  |
Reduction in coal obligation | T |  | 113,000.000 |  |  |  |  |  |  |  |  |
Estimated increase in coal cost |  |  |  | 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 |  |  |  |  |  |  |
Cash used to support bank letter or credit arrangement |  |  |  | 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 |  |  |  |  |  |  |
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 |  |  | $ 43,000,000 |  |  |  |  |
Mine Reclamation Trust | PNM |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  |  | 7,200,000 |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  |  | 8,700,000 |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  | 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 |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  | $ 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 |  |  |  |  |  |  |  |  |  |

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