Debt (Details) (USD $)			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Senior secured revolving credit facility Redetermination	Mar. 31, 2013 Low End of the Range Senior secured revolving credit facility	Mar. 31, 2013 Low End of the Range Senior secured revolving credit facility LIBOR	Mar. 31, 2013 Low End of the Range Senior secured revolving credit facility Prime	Mar. 31, 2013 High End of the Range Senior secured revolving credit facility	Mar. 31, 2013 High End of the Range Senior secured revolving credit facility LIBOR	Mar. 31, 2013 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	653,600,000	562,900,000
Letters of credit issued under the credit facility			23,200,000
Remaining borrowing capacity available under the credit facility			$ 523,200,000
Credit facility borrowing base number of additional yearly redeterminations (in redeterminations)			1