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Commitments and Contingencies - Coal Supply (Details) |  |  |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2017 USD ($) | Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jul. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Feb. 21, 2017 USD ($) | Jul. 19, 2016 USD ($) | Jul. 07, 2016 payment | Jun. 30, 2016 | Apr. 30, 2016 USD ($) | Mar. 31, 2016 USD ($)
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 82,104,000 |  | $ 73,444,000 | $ 82,104,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  | 15,011,000 | 167,471,000 | $ 1,062,000 |  |  |  |  |  |
Funds contributed to mine reclamation trust |  |  |  |  | $ 1,900,000 |  |  |  |  |  |  |  |  |
San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Replacement reclamation bonds |  |  |  |  |  |  |  |  |  | $ 118,700,000 |  |  |  |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | 74,990,000 |  | 67,355,000 | 74,990,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  | 15,011,000 | 167,471,000 | 1,062,000 |  |  |  |  |  |
Regulatory assets |  |  |  | 342,910,000 |  | 368,855,000 | 342,910,000 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Increase in coal mine decommissioning liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net expense |  |  |  |  |  | 4,500,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual funding post-term reclamation trust |  |  |  |  |  | 7,000,000 | 4,300,000 | $ 1,000,000 |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  |  |  | 8,700,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, next fiscal year |  |  |  |  |  | 7,800,000 |  |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  |  |  |  | 8,300,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reclamation trust funding, next fiscal year |  |  |  |  |  | 2,000,000 |  |  |  |  |  |  |  |
Reclamation trust funding, year 2 |  |  |  |  |  | 2,000,000 |  |  |  |  |  |  |  |
Reclamation trust funding, year 3 |  |  |  |  |  | 2,100,000 |  |  |  |  |  |  |  |
Number of annual installments (in payments) | payment |  |  |  |  |  |  |  |  |  |  | 13 |  |  |
Public Service Company of New Mexico [Member] | Surface [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  | 100,000,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Surface [Member] | Loss on long-term purchase commitment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for interim storage costs |  |  |  | 38,800,000 |  | 41,000,000 | 38,800,000 |  |  |  |  |  |  |
Capped amount that can be collected from retail customers for final reclamation |  |  |  |  |  | 100,000,000.0 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Underground [Member] | Loss on long-term purchase commitment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liability for interim storage costs |  |  |  | 11,400,000 |  | 14,000,000 | 11,400,000 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Surface [Member] | Loss on long-term purchase commitment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  | 16,500,000 |  |  | 16,500,000 |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  | 102,000,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Underground [Member] | Loss on long-term purchase commitment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  | 127,800,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  |  | $ 49,000,000 |  | 48,700,000 | $ 49,000,000 |  |  |  |  |  |  |
Estimated increase in coal cost |  |  |  |  |  |  |  |  |  |  |  | 48.00% |  |
NM Capital [Member] | BTMU Term Loan Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | $ 125,000,000.0 |  |  |  | 92,200,000 |  |  |  |  |  |  |  |
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  | $ 82,800,000 |  |  |  |  |
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Funding provided |  | $ 125,000,000.0 | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  | 7.25% |  |  |  |  |  |  |  |  |  |  |  |
Coal mine reclamation bonds to be posted with NMMMD |  |  |  |  |  |  |  |  |  |  |  |  | $ 118,700,000 | $ 161,600,000
Cash used to support bank letter or credit arrangement |  |  | 30,300,000 |  |  |  |  |  |  | $ 30,300,000 |  |  | $ 40,000,000 |
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member] | Subsequent Event [Member] | Westmoreland San Juan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal payment received | $ 9,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  | $ 125,000,000 |  |  | 95,000,000 |  |  |  |  |  |  |  |
Prepayment penalty |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  |  |  |  |  |  |  | $ 85,400,000 |  |  |  |  |

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