Decommissioning and Other Asset Retirement Obligations (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Decommissioning and Other Asset Retirement Obligations Detail [Table]
Beginning balance for the period, as reported	$ 113,138,000	$ 144,525,000	$ 139,835,000	$ 272,815,000
Activity in the period:
Accretion of liability	285,000	631,000	1,306,000	3,789,000
Revisions in estimated cash flows	9,187,000	14,311,000	19,293,000	40,120,000
Settlement of retirement obligations	(23,104,000)	(25,872,000)	(60,928,000)	(183,129,000)
Ending balance at September 30	99,506,000	133,595,000	99,506,000	133,595,000
Amount of asset retirement obligations assumed by buyers of oil and gas properties	$ 3,300,000		$ 122,000,000