Commitments and Contingencies (Commitments and Contingencies Related to the Environment) (Details) (USD $) In Millions, unless otherwise specified										12 Months Ended			12 Months Ended				3 Months Ended
	Dec. 21, 2011 mw	Dec. 21, 2011 San Juan Generating Station [Member]	Oct. 31, 2012 San Juan Generating Station [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 San Juan Generating Station Units 1 and 4 [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 San Juan Generating Station Units 2 and 3 [Member] PNM [Member]	Nov. 08, 2010 Four Corners [Member]	Mar. 31, 2013 Clean Air Act related to Regional Haze [Member] state	Mar. 31, 2013 Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member]	Aug. 06, 2012 Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member] compliance_alternative	Sep. 30, 2012 Mercury Control [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member]	Jan. 31, 2010 Minimum [Member] San Juan Generating Station And Four Corners [Member] Public Service Company of New Mexico [Member] opp	Jan. 31, 2010 Maximum [Member] San Juan Generating Station And Four Corners [Member] Public Service Company of New Mexico [Member] opp	Sep. 30, 2012 Maximum [Member] Mercury Control [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] Clean Air Act, SNCR [Member] San Juan Generating Station Units 1 and 4 [Member] Public Service Company of New Mexico [Member]	Oct. 31, 2012 Subsequent Event [Member] Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] Minimum [Member] Clean Air Act, SNCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member]	Oct. 31, 2012 Subsequent Event [Member] Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] Maximum [Member] Clean Air Act, SNCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 Environmental Protection Agency [Member] WildEarth Guardians filed an action to challenge EPA's rule in the Tenth Circuit [Member] San Juan Generating Station [Member]	Mar. 31, 2013 Environmental Protection Agency [Member] Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 Nuclear Spent Fuel And Waste Disposal [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 Nuclear Spent Fuel And Waste Disposal [Member] Other Deferred Credits [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 Nuclear Spent Fuel And Waste Disposal [Member] Other Deferred Credits [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member]
Public Utilities, Commitments and Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss								$ 75.0											$ 42.8
Public Utilities, Number of Compliance alternatives									2
Public Utilities, Number of States To Address Regional Haze							50
Public Utilities, Potential to emit tons per year of visibility impairing pollution, maximum							250
Estimated Total Capital Cost If Requirement Occured															824	910
Estimated Installation Capital Costs														63.0	85.0	90.0
Estimated Portion of Total Capital Costs if Requirement Occured															105	110
Jointly Owned Utility Plant, Proportionate Ownership Share			46.30%	52.00%
estimated cost of replacing gas fired peaking capacity due to retirement of SJGS units					280
Net book value					290
Public Utilities, Government Standard Emission Limit											0.060	0.070
Public Utilitites, Plant Requirment to Meet Opacity Limit, Percentage								20.00%
Public Utilities, Rule Imposes Opacity Limitation on Certain Fugitive Dust Emissions From Coal and Material Handling Operations								20.00%
Current Annual Mercury Control Costs										0.6
Contingent Estimated Annual Mercury Control Cost													6.1
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, Proposed Seeking Shorter Compliance Period																	5 years
Public Utilities, Number of years after issuance of final determination to achieve compliance with requirements																		3 years
Loss Contingency Accrual, at Carrying Value																				$ 12.6	$ 13.9
Public Utilties, Jointly Owned Utility Plant, Sale of Ownership Percentage						48.00%