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Commitments and Contingencies (Commitments and Contingencies Related to the Environment) (Details) (USD $) In Millions, unless otherwise specified |  | 0 Months Ended | 3 Months Ended |  | 3 Months Ended |  |  |  |  |  |  |  |  | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended
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Dec. 31, 2011 mw | Sep. 21, 2011 EPA [Member] | Mar. 31, 2012 EPA [Member] | Mar. 31, 2012 WildEarth Guardians filed an action to challenge EPA's rule in the Tenth Circuit [Member] San Juan Generating Station [Member] EPA [Member] | Mar. 31, 2012 Clean Air Act related to Regional Haze [Member] | Mar. 31, 2012 Sierra Club Allegations [Member] San Juan Generating Station [Member] | Jan. 31, 2010 Public Service Company of New Mexico [Member] San Juan Generating Station And Four Corners [Member] Minimum [Member] opp | Jan. 31, 2010 Public Service Company of New Mexico [Member] San Juan Generating Station And Four Corners [Member] Maximum [Member] opp | Mar. 31, 2012 Public Service Company of New Mexico [Member] San Juan Generating Station [Member] | Dec. 31, 2011 Public Service Company of New Mexico [Member] San Juan Generating Station [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Nuclear Spent Fuel And Waste Disposal [Member] Palo Verde Nuclear Generating Station [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Nuclear Spent Fuel And Waste Disposal [Member] Palo Verde Nuclear Generating Station [Member] Other Deferred Credits [Member] | Dec. 31, 2011 Public Service Company of New Mexico [Member] Nuclear Spent Fuel And Waste Disposal [Member] Palo Verde Nuclear Generating Station [Member] Other Deferred Credits [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act related to SCR [Member] San Juan Generating Station [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act, SNCR [Member] San Juan Generating Station [Member] Installation Costs [Member] | Sep. 21, 2011 Public Service Company of New Mexico [Member] Clean Air Act, SCR [Member] San Juan Generating Station [Member] Selective_Catalytic_Reduction | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act, SCR [Member] San Juan Generating Station [Member] Operating Costs [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act, SCR [Member] San Juan Generating Station [Member] Operating Costs [Member] Share of project costs to be incurred in 2013 [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act related to Post-Combustion Controls [Member] Four Corners [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Clean Air Act related to Post-Combustion Controls [Member] Four Corners [Member] EPA [Member] | Apr. 06, 2012 Public Service Company of New Mexico [Member] National Ambient Air Quality Standards [Member] San Juan Generating Station And Four Corners [Member] Application for an amendment to its air permit [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Mercury Control [Member] San Juan Generating Station [Member] | May 17, 2010 Public Service Company of New Mexico [Member] Mercury Control [Member] San Juan Generating Station [Member] Maximum [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Sierra Club Allegations [Member] San Juan Generating Station [Member] | Mar. 31, 2012 Public Service Company of New Mexico [Member] Cooling Water Intake Structures [Member] EPA [Member]
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Period of required selective catalytic reduction to be installed |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingency, Estimate of Possible Loss |  |  |  |  |  |  |  |  |  |  | $ 42.8 |  |  |  | $ 77.0 |  |  |  | $ 69.0 |  |  |  | $ 6.0 |  |
Public Utilities, Estimate cost of equipment installation modifications to reduce fugitive emissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 140 |  |  |  |
Public Utilities, Estimate cost of equipment installation modifications to reduce fugitive emissions, percentage share of cost to purchase equipment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 46.30% |  |  |  |
Jointly Owned Utility Plant, Proportionate Ownership Share |  |  |  |  |  |  |  |  | 46.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingency Accrual, at Carrying Value |  |  |  |  |  |  |  |  |  |  |  | 13.9 | 14.5 |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Potential to emit tons per year of visibility impairing pollution, maximum |  |  |  |  | 250 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Number of selective catalytic reduction installations that might need to be installed |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |
Loss Contingency, Range of Possible Loss, Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  | 750.0 |  |  | 18.0 | 85.0 |  |  |  |  |  |  |
Loss Contingency, Range of Possible Loss, Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,000.0 |  |  | 27.0 | 115.0 |  |  |  |  |  |  |
Public Utilities, Government Standard Emission Limit (pounds per MMBTU, parts per million) |  |  |  |  |  |  | 0.060 | 0.070 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingency, Loss in Period |  |  |  |  |  | $ 10.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.6 |  | $ 4.5 |
Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Merucy and Air Toxics Standards | 25 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mercury Removal Rate, Percentage |  |  |  |  |  |  |  |  |  | 0.99 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Original Compliance Period |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Proposed Seeking Shorter Compliance Period |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Number of years after issuance of final determination to achieve compliance with requirements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |
Public Utilites, Number of days to repond to request |  |  | 90 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Proposed rule requires existing facilities to comply with the impingement mortality requirements, maximum time to comply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 years

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- Definition Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Merucy and Air Toxics Standards + References No definition available. + Details
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- Definition Public Utilities, Number of selective catalytic reduction installations that might need to be installed + References No definition available. + Details
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