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Commitments and Contingencies - Coal Supply (Details) |  |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Mar. 31, 2017 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | May 02, 2017 USD ($) | May 01, 2017 USD ($) | Apr. 24, 2017 USD ($) | Feb. 01, 2017 | Aug. 01, 2016 payment
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | $ 66,093,000 |  | $ 73,444,000 | $ 73,444,000 |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | 0 | $ 774,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | 60,425,000 |  | 67,355,000 | 67,355,000 |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | 0 | $ 774,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Increase in coal mine decommissioning liability |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Expense reflected in regulatory disallowances and restructuring costs |  |  |  |  | 4,500,000 | 4,500,000 |  |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  | 6,900,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, next fiscal year |  |  | 8,300,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, third fiscal year |  |  | 8,700,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  | 2,000,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, next fiscal year |  |  | 2,100,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, third fiscal year |  |  | 2,100,000 |  |  |  |  |  |  |  |  |
Number of annual installment payments | payment |  |  |  |  |  |  |  |  |  |  |  | 13
Public Service Company of New Mexico | Surface |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  | 100,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Loss on long-term purchase commitment | Surface |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  | 41,500,000 |  | 41,000,000 | 41,000,000 |  |  |  |  |  |
Final reclamation, capped amount to be collected |  |  | 100,000,000.0 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Loss on long-term purchase commitment | Underground |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  | 14,000,000 |  | 14,000,000 | 14,000,000 |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Loss on long-term purchase commitment | Surface |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  | $ 16,500,000 |  |  |  |  |
Estimate of possible loss |  |  | 102,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Loss on long-term purchase commitment | Underground |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  | 127,800,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | San Juan Generating Station | Coal supply |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | 43,000,000 |  | $ 48,700,000 | $ 48,700,000 |  |  |  |  |  |
Estimated increase in coal cost |  |  |  |  | 48.00% | 48.00% |  |  |  |  |  |
NM Capital | BTMU Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Term loans | $ 125,000,000.0 |  | 82,800,000 |  |  |  |  |  |  |  |  |
NM Capital | San Juan Generating Station | Coal supply |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Interest rate |  | 7.25% |  |  |  |  |  |  |  |  | 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 |  |  |  |  |  |  |  |  |
NM Capital | San Juan Coal Company, Westmoreland | Coal supply |  |  |  |  |  |  |  |  |  |  |  |
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 | 85,400,000 |  |  |  |  |  |  |  |  |
Prepayment penalty |  |  | $ 0 |  |  |  |  |  |  |  |  |
NM Capital | San Juan Coal Company, Westmoreland | Coal supply | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payment proceeds |  |  |  |  |  |  |  | $ 9,600,000 | $ 9,600,000 |  |  |
Scheduled interest payment proceeds |  |  |  |  |  |  |  | $ 2,200,000 | $ 2,200,000 |  |  |
Unaffiliated third party | San Juan Coal Company, Westmoreland | Coal supply | Subsequent event |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount held in a restricted bank account |  |  |  |  |  |  |  |  |  | $ 11,800,000 |  |

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