Debt (Senior Secured Revolving Credit Facilty) (Details) (USD $)			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Senior secured revolving credit facility Redetermination	Mar. 31, 2012 Senior secured revolving credit facility LIBOR	Mar. 31, 2012 Senior secured revolving credit facility Prime	Apr. 13, 2012 Credit Agreement Amendment Senior secured revolving credit facility
Debt obligations
Lender commitments			$ 1,200,000,000
Credit facility maximum borrowing base			1,287,500,000			1,400,000,000
Letters of credit issued under the facility			23,200,000
Remaining borrowing capacity available under the agreement			1,176,800,000
Outstanding borrowings under credit facility	$ 0	$ 531,500,000
Credit facility borrowing base number of additional yearly redeterminations			1
Debt instrument basis spread on variable rate low end of range (as a percent)				1.50%	0.50%
Debt Instrument basis spread on variable rate high end of range (as a percent)				2.50%	1.50%
Annual commitment fee on the unused portion of the credit agreement low end of range (as a percent)			0.35%
Annual commitment fee on the unused portion of the credit agreement high end of range (as a percent)			0.50%