Variable Interest Entities (Details) (USD $)	3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended				3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2013 Public Service Company of New Mexico [Member]	Jun. 30, 2012 Public Service Company of New Mexico [Member]	Jun. 30, 2013 Public Service Company of New Mexico [Member]	Jun. 30, 2012 Public Service Company of New Mexico [Member]	Mar. 31, 2013 Public Service Company of New Mexico [Member]	Dec. 31, 2012 Public Service Company of New Mexico [Member]	Jun. 30, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2012 Valencia [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2012 Valencia [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 Valencia [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Palo Verde Nuclear Generating Station [Member] Property Lease Guarantee [Member] Maximum [Member] Public Service Company of New Mexico [Member]	May 30, 2008 Purchased Through May 30, 2028 [Member] Valencia [Member] Public Service Company of New Mexico [Member] MW	Jun. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Mar. 31, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Jun. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 Delta [Member] Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Number of mega watts purchased (in megawatts)																						145
Long Term Contract For Purchase of Electric Power Fixed Costs													$ 4,700,000	$ 4,600,000	$ 9,400,000	$ 9,300,000		$ 1,600,000	$ 3,200,000
Long Term Contract For Purchase of Electric Power Variable Charges													200,000		300,000	300,000		400,000	600,000
Variable Interest Entity, Statement Of Operation [Abstract]
Operating revenues													4,922,000	4,888,000	9,697,000	9,559,000
Operating expenses													(1,349,000)	(1,433,000)	(2,920,000)	(2,839,000)
Earnings Attributable to non-controlling interest	3,573,000	3,455,000	6,777,000	6,720,000			3,573,000	3,455,000	6,777,000	6,720,000			3,573,000	3,455,000	6,777,000	6,720,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets	393,157,000		393,157,000			442,191,000	384,105,000		384,105,000			387,689,000	3,124,000		3,124,000		3,655,000
Net property, plant and equipment													76,541,000		76,541,000		77,953,000
Assets	5,403,906,000	5,252,589,000	5,403,906,000	5,252,589,000		5,372,583,000	4,185,189,000		4,185,189,000			4,163,907,000	79,665,000		79,665,000		81,608,000
Current liabilities					444,590,000	434,103,000					196,239,000	242,751,000	720,000		720,000		765,000
Owners' equity - non-controlling interest					78,945,000	80,843,000					78,945,000	80,843,000	78,945,000		78,945,000		80,843,000
Operating Leases, Fixed Rate Percentage of Renewal Options After Original Lease Term									50.00%
Operating Leases, Renewal Options After Original Lease Term (in years)									2 years
Operating Lease, Percentage of Unit 2 Capacity That May Be Extended									14.00%
Operating Leases, Extended Lease Term Option (in years)									6 years
Operating Leases, Future Minimum Payments Due																				65,900,000
Loss Contingency, Range of Possible Loss, Portion Not Accrued																					160,400,000
Operating Leases, Future Minimum Payments Due, Next Six Months																				26,000,000
Long Term Contract for Purchase of Electric Power Aggregate Amount of Contract Remaining																		42,200,000	42,200,000
Payments to Acquire Businesses, Gross																										23,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt																							17,700,000		17,700,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt																							3,200,000		3,200,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets																							25,400,000		25,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment																							21,700,000		21,700,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities																							19,600,000		19,600,000
Revenues																							2,200,000		4,000,000
Net Earnings	$ 27,678,000	$ 21,512,000	$ 38,304,000	$ 38,592,000			$ 26,124,000	$ 16,885,000	$ 37,693,000	$ 34,697,000														$ 100,000	$ 300,000