Debt (Senior Secured Revolving Credit Facilty) (Details) (USD $)			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Senior secured revolving credit facility Redetermination	Jun. 30, 2012 Senior secured revolving credit facility LIBOR	Jun. 30, 2012 Senior secured revolving credit facility Prime	Jun. 30, 2012 Low End of the Range Senior secured revolving credit facility	Jun. 30, 2012 High End of the Range Senior secured revolving credit facility
Debt obligations
Credit facility maximum borrowing base			$ 1,400,000,000
Lender commitments			1,200,000,000
Loans Payable Basis Spread On Variable Rate Low End Of Range				1.50%	0.50%
Debt Instrument basis spread on variable rate high end of range (as a percent)				2.50%	1.50%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage (as a percent)						0.35%	0.50%
Outstanding borrowings under credit facility	400,500,000	531,500,000
Letters of credit issued under the facility			23,200,000
Remaining borrowing capacity available under the credit facility			$ 776,300,000
Credit facility borrowing base number of additional yearly redeterminations (in redeterminations)			1