Natural Gas and Oil Producing Activities (Analysis of Changes in the Standardized Measure) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Natural Gas And Oil Producing Activities Abstract
Standardized measure, beginning of year	$ 3,013,750	$ 1,801,818	$ 2,109,262
Sales and transfers of natural gas and oil produced, net of production costs	(1,632,156)	(1,516,571)	(1,330,256)
Net changes in prices and production costs	(381,131)	706,062	(1,321,404)
Extensions, discoveries, and other additions, net of future production and development costs	1,163,992	1,205,464	976,449
Acquisition of reserves in place	30	0	1,878
Sales of reserves in place	(11,761)	(6,269)	(4,430)
Revisions of previous quantity estimates	34,221	324,284	88,261
Accretion of discount	426,245	230,355	302,439
Net change in income taxes	(103,643)	(746,971)	413,399
Changes in estimated future development costs	70,492	(10,558)	204,005
Previously estimated development costs incurred during the year	564,894	353,560	218,625
Changes in production rates (timing) and other	305,867	672,576	143,590
Standardized measure, end of year	$ 3,450,800	$ 3,013,750	$ 1,801,818