Derivative Activities (Details 3) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	$ 108,091	$ 173,128
Liabilities Carrying Value	0	0
Net Carrying Value	108,091	173,128
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	20,180	17,259
Liabilities Carrying Value	(67,181)	(72,701)
Net Carrying Value	(47,001)	(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	103,907	164,933
Liabilities Carrying Value	0	0
Net Carrying Value	103,907	164,933
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	6,430	17,259
Liabilities Carrying Value	(15,830)	(12,052)
Net Carrying Value	(9,400)	5,207
Swap [Member]
Derivatives that qualify for cash flow hedge accounting :
Assets Carrying Value	4,184	0
Liabilities Carrying Value	0	0
Net Carrying Value	4,184	0
Derivatives that do not qualify for hedge accounting :
Assets Carrying Value	13,750	0
Liabilities Carrying Value	0	0
Net Carrying Value	13,750	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	(51,351)	(60,297)
Net Carrying Value	$ (51,351)	$ (60,297)