Decommissioning and Other Asset Retirement Obligations (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Decommissioning and Other Asset Retirement Obligations Detail [Table]
Beginning balance for the period, as reported	$ 126,844,000	$ 230,834,000	$ 139,835,000	$ 272,815,000
Activity in the period:
Accretion of liability	501,000	1,264,000	1,021,000	3,158,000
Revisions in estimated cash flows	7,876,000	16,045,000	10,106,000	25,809,000
Settlement of retirement obligations	(22,083,000)	(103,618,000)	(37,824,000)	(157,257,000)
Ending balance at June 30	113,138,000	144,525,000	113,138,000	144,525,000
Amount of asset retirement obligations assumed by buyers of oil and gas properties		$ 72,700,000		$ 118,700,000