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Commitments and Contingencies - Litigation and Rulemaking (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 USD ($) | Sep. 30, 2015 USD ($) | Dec. 21, 2011 mw
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Minimum megawatt capacity from coal and oil-fired electric generating units under jurisdiction of the Mercury and Air Toxics Standards (in megawatts) | mw |  |  | 25
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Mercury removal rate | 99.00% | 99.00% |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Mercury Control [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Current annual mercury control costs |  | $ 0.7 |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Maximum [Member] | Mercury Control [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Contingent estimated annual mercury control cost |  | $ 6.6 |
Public Service Company of New Mexico [Member] | Four Corners Units 4 and 5 (Coal) [Member] | Clean Air Act Lawsuit [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Litigation settlement, amount | $ 1.5 |  |
Litigation settlement, expected capital spend, environmental | $ 6.2 |  |
Public utilities, ownership percentage | 13.00% | 13.00% |

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