Natural Gas and Oil Producing Activities (Unaudited) (Analysis of Changes in the Standardized Measure) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Natural Gas And Oil Properties [Abstract]
Standardized measure, beginning of year	$ 3,450,800	$ 3,013,750	$ 1,801,818
Sales and transfers of natural gas and oil produced, net of production costs	(1,443,606)	(1,632,156)	(1,516,571)
Net changes in prices and production costs	(2,604,591)	(381,131)	706,062
Extensions, discoveries, and other additions, net of future production and development costs	549,601	1,163,992	1,205,464
Acquisition of reserves in place		30
Sales of reserves in place	(157,108)	(11,761)	(6,269)
Revisions of previous quantity estimates	(1,109,409)	34,221	324,284
Accretion of Discount	480,315	426,245	230,355
Net change in income taxes	1,079,158	(103,643)	(746,971)
Changes in estimated future development costs	2,475,470	70,492	(10,558)
Previously estimated development costs incurred during the year	61,949	564,894	353,560
Changes in production rates (timing) and other	(731,914)	305,867	672,576
Standardized measure, end of year	$ 2,050,665	$ 3,450,800	$ 3,013,750