Oil And Natural Gas Properties (Details) (USD $)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2012	Sep. 30, 2012	Sep. 30, 2011
Oil And Natural Gas Properties [Line Items]
Discount rate used to calculate ceiling, percentage				10.00%
Impairment of oil and gas properties	$ 0	$ 0	$ 115,874,000	$ 115,874,000	$ 0
Impairment of oil and gas properties, net of taxes			72,100,000	72,100,000
Effects of cash flow hedges	$ 38,400,000		$ 32,500,000	$ 38,400,000
Jul'12 - Dec'12 [Member]
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe			2.9
Jan'13 - Dec'13 [Member]
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe	6.9		4.5	6.9
Oct'12 - Dec'12 [Member]
Oil And Natural Gas Properties [Line Items]
Oil and gas production volume covered by hedges that affect ceiling determination, MMBoe	1.3			1.3