Natural Gas and Oil Producing Activities (Unaudited) (Results of Operations from Natural Gas and Oil Producing Activities) (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]
Sales	$ 1,948,222	$ 2,100,488	$ 1,890,444
Production (lifting) costs	(505,271)	(469,153)	(376,939)
Depreciation, depletion and amortization	(765,192)	(666,107)	(561,003)
Impairment of natural gas and oil properties	(1,939,734)
Results of operations - income before income taxes	(1,261,975)	965,228	952,502
Provision (benefit) for income taxes	(502,690)	376,049	371,281
Results of operations	$ (759,285)	$ 589,179	$ 581,221