Summary of Business and Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Public Service Company of New Mexico [Member]	Dec. 31, 2011 Public Service Company of New Mexico [Member]	Dec. 31, 2010 Public Service Company of New Mexico [Member]	Dec. 31, 2012 Texas-New Mexico Power Company [Member]	Dec. 31, 2011 Texas-New Mexico Power Company [Member]	Dec. 31, 2010 Texas-New Mexico Power Company [Member]	Jan. 04, 2012 Optim Energy [Member]	Dec. 31, 2011 Optim Energy [Member]	Sep. 23, 2011 Optim Energy [Member]	Sep. 22, 2011 Optim Energy [Member]	Dec. 31, 2012 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] Operating_Leases	Dec. 31, 1986 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] Operating_Leases	Dec. 31, 2004 EIP Transmission Line [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2003 EIP Transmission Line [Member] Public Service Company of New Mexico [Member]	Dec. 31, 1985 EIP Transmission Line [Member] Public Service Company of New Mexico [Member] Operating_Leases	Dec. 31, 2012 10.3% Lessor Notes [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 10.15% Lessor Notes [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2011 10.25% EIP Lessor Note Maturing in 2012 [Member] Public Service Company of New Mexico [Member]	Dec. 31, 2012 10.25% EIP Lessor Note Maturing in 2012 [Member] EIP Transmission Line [Member] Public Service Company of New Mexico [Member]
Accounting Policies Disclosures [Line Items]
Equity method investment,ownership percentage (as a percent)													50.00%
Allowance for funds used during construction, capitalized interest				$ 7,300,000	$ 1,500,000	$ 4,600,000	$ 1,300,000	$ 1,200,000	$ 200,000
Debt, weighted average interest rate (as a percent)	6.60%	6.60%	6.70%
Interest costs incurred, capitalized	1,200,000	500,000	800,000	800,000	200,000	400,000
Number of operating leases (in ones)														8	11			2
Notes receivable, stated percentage rate (as a percent)																			10.30%	10.15%		10.25%
Ownership percentage acquired (as a percent)																40.00%	60.00%
Other investments	9,139,000	12,207,000		4,034,000	2,900,000		281,000	271,000													2,200,000
Impairment losses on securities held in the NDT				4,800,000	12,500,000	5,000,000
Cost method investment, ownership percentage (as a percent)												1.00%
Ownership percentage, sold (as a percent)										1.00%
Cost Method Investments										0
Equity Method Investments											$ 0