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Commitments and Contingencies - Coal Supply (Details) |  |  |  |  | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 05, 2018 T | Jun. 29, 2018 USD ($) | Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2018 USD ($) | May 22, 2018 USD ($) | Feb. 01, 2018 | Jun. 30, 2017 | Feb. 01, 2017 | Jul. 19, 2016 USD ($) | Jul. 07, 2016 payment | Mar. 31, 2016 USD ($)
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected number of additional monitoring wells |  |  |  |  | 38 |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  | $ 53,725,000 | $ 47,358,000 |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | 65,598,000 | 27,036,000 | $ 15,011,000 |  |  |  |  |  |  |  |  |
Funds contributed to mine reclamation trust |  |  |  |  | 2,300,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  | 42,433,000 | 39,904,000 |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | 66,339,000 | 27,036,000 | 15,011,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  | 4,500,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Increase in coal mine decommissioning liability | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change |  |  |  |  |  |  |  |  | $ 2,500,000 |  |  |  |  |  |  |
Public Service Company of New Mexico | Loss on long-term purchase commitment | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual funding post-term reclamation trust |  |  |  |  | 10,000,000 | 5,800,000 | $ 7,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, next fiscal year |  |  |  |  | 8,900,000 |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  |  |  | 10,200,000 |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  |  | 10,900,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, next fiscal year |  |  |  |  | 2,300,000 |  |  |  |  |  |  |  |  |  |  |
Number of annual installments (in payments) | payment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13 |
Public Service Company of New Mexico | Surface | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capped amount that can be collected from retail customers for final reclamation |  |  |  |  | 100,000,000 |  |  |  |  |  |  |  |  |  |  |
Liability for interim storage costs |  |  |  |  | 70,100,000 | 41,400,000 |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Surface | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  | 29,800,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Underground and Surface | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  | 39,200,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Underground | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for interim storage costs |  |  |  |  | 23,200,000 | 14,700,000 |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Underground | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  | 9,400,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  | $ 16,500,000 |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | 103,200,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Underground | Loss on long-term purchase commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  | 39,700,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  |  | 26,300,000 | $ 26,300,000 |  |  |  |  |  |  |  |  |  |
Reduction in coal obligation | T | 111,668 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated increase in coal cost |  |  |  |  |  |  |  |  | 6.90% |  |  | 51.00% |  |  |  |
NM Capital | BTMU Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchased face amount |  |  |  |  |  |  |  |  |  | $ 43,000,000 |  |  |  |  |  |
NM Capital | San Juan Generating Station | Coal Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Funding provided |  |  | $ 125,000,000.0 | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  |  |  |  |  |  | 12.25% |  | 9.25% |  |  |
Coal mine reclamation bonds to be posted with NMMMD |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 118,700,000
Cash used to support bank letter or credit arrangement |  |  |  | $ 30,300,000 |  |  |  |  |  |  |  |  |  | $ 30,300,000 |  |
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepayment penalty |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |
Repurchased face amount |  |  |  |  |  |  |  |  |  | $ 50,100,000 |  |  |  |  |  |
Four Corners Coal Supply Arbitration | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for legal settlements |  | $ 4,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Four Corners Coal Supply Arbitration, Period March 1, 2018 through June 30, 2018 | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for legal settlements |  | $ 1,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |

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