Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
ARO Operating Costs
Accretion	$ 1,998	$ 2,124	$ 4,017	$ 4,296
Depreciation	1,863	1,853	3,727	3,395
Total	3,861	3,977	7,744	7,691
Asset Retirement Obligations
Balance at beginning of period	155,402	162,591	153,979	162,730
Liabilities incurred	45	278	45	278
Liabilities settled	(798)	(3,632)	(1,419)	(5,964)
Accretion expense	1,998	2,124	4,017	4,296
Revisions down, net	(6,234)	(628)	(6,209)	(607)
Balance at end of period	$ 150,413	$ 160,733	$ 150,413	$ 160,733