Asset Retirement Obligations (Asset Retirement Obligations) (Details) (USD $) In Thousands	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010
Asset Retirement Obligations
ARO liability, January 1:	$ 69,265		$ 56,404
Accretion of discount	2,598		2,138
Liability incurred	12,674	[1]	2,901	[1]
Liability settled	(831)		(622)
Revision of estimates	9,096	[2]	3,822	[2]
ARO liability, September 30:	92,802		64,643
Less current portion	2,074		1,617
Total long-term ARO liability	$ 90,728		$ 63,026

[1] Liability incurred in 2011 includes estimates for new wells acquired.

[2] Plugging liability estimates were revised upward in both 2011 and 2010 due to increases in the cost of services used to plug wells over the preceding years.