Natural Gas and Oil Producing Activities (Results of Operations from Natural Gas and Oil Producing Activities) (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
Sales	$ 2,100,488	$ 1,890,444	$ 1,593,231
Production (lifting) costs	(469,153)	(376,939)	(259,588)
Depreciation, depletion and amortization	(666,107)	(561,003)	(474,014)
Impairment of natural gas and oil properties	0	0	(907,812)
Results of operations - income before income taxes	965,228	952,502	(48,183)
Provision (benefit) for income taxes	376,049	371,281	(15,650)
Results of operations	$ 589,179	$ 581,221	$ (32,533)