Variable Interest Entities; Acquisition (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended						3 Months Ended		9 Months Ended						3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Public Service Company of New Mexico [Member]	Sep. 30, 2014 Public Service Company of New Mexico [Member] Delta [Member]	Jul. 17, 2014 Public Service Company of New Mexico [Member] Delta [Member]	Sep. 30, 2014 Public Service Company of New Mexico [Member] Palo Verde Nuclear Generating Station [Member]	Dec. 31, 2013 Public Service Company of New Mexico [Member] Palo Verde Nuclear Generating Station [Member]	Sep. 30, 2014 Public Service Company of New Mexico [Member] Palo Verde Nuclear Generating Station [Member] Property Lease Guarantee [Member] Maximum [Member]	Sep. 30, 2014 Valencia [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2014 Valencia [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	Oct. 08, 2013 Valencia [Member] Public Service Company of New Mexico [Member]	May 30, 2008 Valencia [Member] Public Service Company of New Mexico [Member] Purchased Through May 30, 2028 [Member] MW	Sep. 30, 2014 Variable Interest Entity, Not Primary Beneficiary [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2014 Delta [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2014 Delta [Member] Public Service Company of New Mexico [Member]	Sep. 30, 2013 Delta [Member] Public Service Company of New Mexico [Member]
Variable Interest Entity [Line Items]
Number of mega watts purchased (in megawatts)																		158
Long Term Contract For Purchase of Electric Power Fixed Costs												$ 4,800,000	$ 4,800,000	$ 14,400,000	$ 14,100,000					$ 300,000	$ 1,600,000	$ 3,500,000	$ 4,800,000
Long Term Contract For Purchase of Electric Power Variable Charges												300,000	700,000	1,000,000	1,000,000					100,000	700,000	600,000	1,300,000
Variable Interest Entity, Statement Of Operation [Abstract]
Operating revenues												5,061,000	5,453,000	15,300,000	15,150,000
Operating expenses												(1,360,000)	(1,326,000)	(4,160,000)	(4,246,000)
Earnings attributable to non-controlling interest	3,701,000	4,127,000	11,140,000	10,904,000								3,701,000	4,127,000	11,140,000	10,904,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets	466,326,000		466,326,000		401,539,000							3,435,000		3,435,000		2,658,000
Net property, plant, and equipment												73,024,000		73,024,000		75,137,000
Total assets	5,709,209,000	5,428,814,000	5,709,209,000	5,428,814,000	5,500,210,000							76,459,000		76,459,000		77,795,000
Current liabilities	700,292,000		700,292,000		492,671,000							1,039,000		1,039,000		766,000
Owners’ equity – non-controlling interest	75,420,000		75,420,000		77,029,000							75,420,000		75,420,000		77,029,000
Long term contract option to purchase, ownership percentage												50.00%
Long term contract option to purchase, purchase price - percentage of adjusted NBV												50.00%
Long term contract option to purchase, purchase price - percentage of FMV												50.00%
Long term contract option to purchase, number of days to set FMV												60 days
Long term contract option to purchase, estimated purchase price																	85,000,000
Long term contract option to purchase, approximate approval period												15 months
Operating Leases, Renewal Options After Original Lease Term (in years)			6 years			2 years
Operating Leases, Extended Lease Term Option (in years)						6 years
Operating Leases, Future Minimum Payments Due									20,300,000
Operating leases, future minimum payments due, renewal term																			145,200,000
Loss Contingency, Range of Possible Loss, Portion Not Accrued											123,800,000.0
Operating Leases, Future Minimum Payments Due, Next Six Months									11,800,000	26,000,000
Period of Long Term Contract For Purchase of Electric Power Fixed Costs																				20 years
Payments to Acquire Businesses, Gross							22,800,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt							14,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment								58,100,000
Business Combination, recognized identifiable assets acquired, accumulated amort								23,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets								3,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets								3,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities								300,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities								$ 3,400,000