Debt (Senior Secured Revolving Credit Facilty) (Details) (USD $)			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Senior secured revolving credit facility Redetermination	Sep. 30, 2012 Low End of the Range Senior secured revolving credit facility	Sep. 30, 2012 Low End of the Range Senior secured revolving credit facility LIBOR	Sep. 30, 2012 Low End of the Range Senior secured revolving credit facility Prime	Sep. 30, 2012 High End of the Range Senior secured revolving credit facility	Sep. 30, 2012 High End of the Range Senior secured revolving credit facility LIBOR	Sep. 30, 2012 High End of the Range Senior secured revolving credit facility Prime
Debt obligations
Credit facility maximum borrowing base			$ 1,400,000,000
Lender commitments			1,200,000,000
Loans Payable Basis Spread On Variable Rate					1.50%	0.50%		2.50%	1.50%
Line of credit facility, unused capacity, commitment fee (as a percent)				0.35%			0.50%
Outstanding borrowings under credit facility	510,000,000	531,500,000
Letters of credit issued under the facility			23,200,000
Remaining borrowing capacity available under the credit facility			$ 666,800,000
Credit facility borrowing base number of additional yearly redeterminations (in redeterminations)			1