Variable Interest Entities (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	May 30, 2008 mw
Variable Interest Entity [Line Items]
Operating Leases, renewal options after original lease term		2 years
Variable Interest Entity, Statement Of Operation [Abstract]
Earnings Attributable to non-controlling interest		$ 14,047,000	$ 13,563,000	$ 11,890,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets		462,819,000	543,880,000
Total Assets		5,204,613,000	5,225,083,000	5,359,921,000
Current liabilities		373,268,000	520,124,000
Owners' equity - non-controlling interest		(82,423,000)	(85,177,000)
Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Operating Leases, Currently active with trusts		8
Operating Leases, trust lessors		8
Operating Leases, Different institutional investors of trust lessors		5
Operating Leases, renewal options after original lease term		2
Operating Leases, Extended lease term option		6
Operating leases, future minimum payments due		300,356,000
Long term contract for purchase of electric power fixed costs		6,000,000	6,000,000	6,400,000
Long term contract for purchase of electric power variable charges		1,500,000	400,000	400,000
Variable Interest Entity, Statement Of Operation [Abstract]
Earnings Attributable to non-controlling interest		14,047,000	13,563,000	11,890,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets		459,621,000	384,038,000
Total Assets		4,095,287,000	3,875,573,000
Current liabilities		290,563,000	394,961,000
Owners' equity - non-controlling interest		(82,423,000)	(85,177,000)
Operating Leases, Future Minimum Payments Due, Current		43,562,000
Long Term Contract for Purchase of Electric Power Aggregate Amount of Contract Remaining		51,100,000
Valencia [Member] | Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Number of mega watts purchased (in megawatts)					145
Power plant total construction costs					90,000,000
Long trm contract for purchase of power term of contract	20 years
Long term contract for purchase of electric power fixed costs		18,300,000	17,900,000	17,700,000
Long term contract for purchase of electric power variable charges		1,400,000	1,100,000	500,000
Variable Interest Entity, Statement Of Operation [Abstract]
Electric Revenue		19,720,000	18,907,000	18,175,000
Operating expenses		(5,673,000)	(5,344,000)	(6,285,000)
Earnings Attributable to non-controlling interest		(14,047,000)	(13,563,000)	(11,890,000)
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets		2,405,000	2,372,000
Net property, plant and equipment		80,785,000	83,617,000
Total Assets		83,190,000	85,989,000
Current liabilities		767,000	812,000
Owners' equity - non-controlling interest		82,423,000	85,177,000
Valencia [Member] | Maximum [Member] | Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Jointly owned utility plant, option to purchase proportionate ownership share		50.00%
PVNGS [Member] | Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Operating leases, future minimum payments due		105,100,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Loss contingency, range of possible loss, portion not accrued		178,300,000
Palo Verde Nuclear Generating Station [Member] | PVNGS [Member] | Public Service Company of New Mexico [Member]
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Operating Leases, Future Minimum Payments Due, Current		$ 26,000,000	$ 26,000,000