Goodwill and Other Intangible Assets (Details) (USD $)	12 Months Ended			12 Months Ended				12 Months Ended			12 Months Ended
	Mar. 31, 2011	Mar. 31, 2010	Jun. 30, 2005	Dec. 31, 2008 Customer Lists [Member]	Dec. 31, 2007 Customer Lists [Member] years	Dec. 31, 2010 Customer Lists [Member]	Jun. 30, 2005 Customer Lists [Member]	Dec. 31, 2008 Trade Names [Member]	Dec. 31, 2010 Trade Names [Member]	Jun. 30, 2005 Trade Names [Member]	Dec. 31, 2008 Public Service Company of New Mexico [Member]	Dec. 31, 2008 Texas-New Mexico Power Company [Member]	Dec. 31, 2008 First Choice [Member]
Schedule of Goodwill and Other Intangible Assets [Line Items]
Goodwill			$ 102,800,000
Business Acquisition, Purchase Price Allocation, Intangible Assets			79,300,000
Business Acquisition, Purchase Price Allocation, Intangible Assets Not Amortizable										68,800,000
Business Acquisition, Purchase Price Allocation, Amortizable Intangible Assets							10,500,000
Finite-Lived Intangible Assets, Useful Life					8
Goodwill, Impairment Loss	0	0									51,100,000	34,500,000	88,800,000
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	0	0						42,600,000
Impairment of Intangible Assets, Finite-lived				4,800,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)									26,200,000
Finite-Lived Intangible Assets, Gross						$ 5,700,000