Intangible Assets (Details 3) (USD $)							12 Months Ended
	Dec. 31, 2009	Dec. 31, 2009 CENG	Nov. 06, 2009 CENG	Dec. 31, 2011 Unamortized energy contracts, net	Dec. 31, 2010 Unamortized energy contracts, net	Dec. 31, 2009 Unamortized energy contracts, net CENG	Dec. 31, 2011 Unamortized energy contracts, net Nonregulated businesses	Dec. 31, 2010 Unamortized energy contracts, net Nonregulated businesses	Dec. 31, 2009 Unamortized energy contracts, net Nonregulated businesses
Unamortized Energy Contracts.
Fair value of power purchase agreement with CENG	$ 800,000,000	$ 800,000,000	$ 800,000,000			$ 800,000,000
Carrying Amount				(1,454,900,000)	(1,360,900,000)
Accumulated Amortization				1,152,500,000	1,473,800,000
Net (Liability) Asset				(302,400,000)	112,900,000
Amortization expense (income)							395,400,000	106,800,000	(353,100,000)
Estimated amortization expense (income)
2012							(65,200,000)
2013							(81,200,000)
2014							(71,300,000)
2015							(65,600,000)
2016							$ (16,400,000)