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Commitments and Contingencies - Coal Supply (Details) |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) | Nov. 01, 2016 USD ($) | Oct. 21, 2016 USD ($) | Aug. 31, 2016 USD ($) | Aug. 30, 2016 USD ($) | Aug. 01, 2016 USD ($) payment | Jul. 19, 2016 USD ($) | Apr. 30, 2016 USD ($)
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | $ 89,746,000 |  | $ 89,746,000 |  | $ 82,104,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | 16,451,000 | $ 0 | 17,225,000 | $ 1,744,000 |  |  |  |  |  |  |  |
Mine reclamation trust requirement funded |  |  | 1,900,000 |  |  |  |  |  |  |  |  |  |  |
San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Replacement Coal Mine Reclamation Bonds posted by alternate surety companies |  |  |  |  |  |  |  |  |  |  |  |  | $ 118,700,000 |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | 81,419,000 |  | 81,419,000 |  | 74,990,000 |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | 16,451,000 | $ 0 | 17,225,000 | $ 1,744,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Increase in coal mine decommissioning liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense reflected in regulatory disallowances and restructuring costs |  |  | 4,800,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Required contribution to Reclamation Trust, current fiscal year |  |  | 3,500,000 |  | 3,500,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, next fiscal year |  |  | 4,700,000 |  | 4,700,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, third fiscal year |  |  | 5,100,000 |  | 5,100,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Mine Reclamation Trust [Member] | Four Corners [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, next fiscal year |  |  | 2,000,000 |  | 2,000,000 |  |  |  |  |  |  |  |  |
Anticipated funding to Reclamation Trust, third fiscal year |  |  | 2,000,000 |  | 2,000,000 |  |  |  |  |  |  |  |  |
Number of annual installment payments | payment |  |  |  |  |  |  |  |  |  |  |  | 13 |  |
Public Service Company of New Mexico [Member] | Surface [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  | 100,000,000 |  | 100,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | Surface [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  | 41,400,000 |  | 41,400,000 |  | 38,800,000 |  |  |  |  |  |  |
Final reclamation, capped amount to be collected |  |  |  |  | 100,000,000.0 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Loss on long-term purchase commitment [Member] | Underground [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency accrual |  |  | 13,900,000 |  | 13,900,000 |  | 11,400,000 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on long-term purchase commitment [Member] | Surface [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  | 16,500,000 |  |  |  |  |  |  |
Estimate of possible loss |  |  | 103,000,000 |  | 103,000,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Loss on long-term purchase commitment [Member] | Underground [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  | 129,400,000 |  | 129,400,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other current assets |  |  | $ 54,100,000 |  | $ 54,100,000 |  | 49,000,000 |  |  |  |  |  |  |
Estimated increase in coal cost |  |  | 40.00% |  | 40.00% |  |  |  |  |  |  |  |  |
NM Capital [Member] | BTMU Term Loan Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans | $ 125,000,000.0 |  | $ 107,800,000.0 |  | $ 107,800,000.0 |  |  |  |  |  |  |  |  |
NM Capital [Member] | BTMU Term Loan Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt-to-capital ratio covenant | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |
NM Capital [Member] | San Juan Generating Station [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |
Initial interest rate |  | 7.25% |  |  |  |  |  |  |  |  |  |  |  |
Requirement to post reclamation bonds |  |  |  |  |  |  | 161,600,000 |  |  |  |  |  |  | $ 118,700,000
Cash used to support bank letter or credit arrangement |  |  |  |  |  |  | $ 40,000,000 |  |  |  |  |  | $ 30,300,000 |
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | 110,000,000 |  | 110,000,000 |  |  |  |  |  |  |  |  |
Scheduled principal payment proceeds |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000 |  |
Prepayment penalty |  |  | $ 0 |  | $ 0 |  |  |  |  |  |  |  |  |
Cash used to support bank letter or credit arrangement |  |  |  |  |  |  |  |  |  | $ 30,300,000 | $ 40,000,000 |  |  |
NM Capital [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Scheduled principal payment proceeds |  |  |  |  |  |  |  | $ 15,000,000 |  |  |  |  |  |
Scheduled interest payment proceeds |  |  |  |  |  |  |  | $ 2,300,000 |  |  |  |  |  |
Unaffiliated third party [Member] | San Juan Coal Company, Westmoreland [Member] | Coal Supply [Member] | Subsequent event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount held in a restricted bank account |  |  |  |  |  |  |  |  | $ 17,300,000 |  |  |  |  |

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