Asset Retirement Obligations (Schedule Of Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]
ARO liability, January 1:	$ 146,159		$ 96,446
Accretion of discount	1,521		1,101
Liability incurred	899		1,204
Liability settled	(1,688)		(1,052)
Revision of estimates	(11,299)	[1]	(5,050)	[1]
ARO liability, March 31:	135,592		92,649
Less current portion	2,917		2,904
Total long-term ARO	$ 132,675		$ 89,745

[1] Plugging liability estimates were revised in both March 2013 and March 2012 for updates in the cost of services used to plug wells over the preceding year. Although cost per well increased, a decrease in the inflation factor resulted in a decrease in estimated cost.