Asset Retirement Obligations (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2011	$ 46,566
Obligations incurred with development activities	13,995
Accretion Expense	1,643
Obligations discharged with disposal of properties and asset retirements	(2,098)
Balance at June 30, 2012	60,106
Asset Retirement Obligation, Current	(250)
Asset Retirement Obligations, Noncurrent	$ 59,856	$ 46,316	[1]

[1] Derived from audited 2011 balance sheet.