Derivative Activities (Details 3) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	$ 166,371	$ 173,128
Liabilities Carrying Value	0	0
Net Carrying Value	166,371	173,128
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	42,315	17,259
Liabilities Carrying Value	(25,077)	(72,701)
Net Carrying Value	17,238	(55,442)
Basis Swap [Member]
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	0	0
Liabilities Carrying Value	0	(352)
Net Carrying Value	0	(352)
Collars [Member]
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	147,067	164,933
Liabilities Carrying Value	0	0
Net Carrying Value	147,067	164,933
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	5,434	17,259
Liabilities Carrying Value	(4,340)	(12,052)
Net Carrying Value	1,094	5,207
Swap [Member]
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	19,304	0
Liabilities Carrying Value	0	0
Net Carrying Value	19,304	0
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	36,881	0
Liabilities Carrying Value	0	0
Net Carrying Value	36,881	0
Collars discontinued operations [Member]
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	0	8,195
Liabilities Carrying Value	0	0
Net Carrying Value	0	8,195
Call Option [Member]
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	0	0
Liabilities Carrying Value	(20,737)	(60,297)
Net Carrying Value	$ (20,737)	$ (60,297)