Acquisitions (Details Textual) (USD $)	12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 Acquisition From Energy Contractors [Member]	Dec. 31, 2011 Peak [Member]	Jul. 31, 2011 Peak [Member]	Dec. 31, 2010 Peak [Member]
Acquisitions and Divestitures (Textual) [Abstract]
Consideration paid in cash	$ 681,300,000					$ 65,000,000
Remaining percentage of equity interest of Peak acquired							50.00%
Common stock of acquired company previously owned	0	0	0				38,100,000
Acquired price per common share	$ 22.12
Acquisition-related transaction costs	7,000,000
Long-term debt	124,792,000
Total asset purchase price				53,400,000
Goodwill	334,992,000			4,200,000
Equity Interest acquired	50.00%					50.00%
Gain on acquisition of equity method investment					13,100,000
Superior contributed revenues	$ 1,200,000,000