Oil and Gas Operations (Unaudited) (Results of Operations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Gross revenues	$ 1,167,183	[1]	$ 944,908	[1]	$ 957,371	[1]
Production (lifting costs)	306,375		257,045		224,901
Exploration expense	19,363		13,110		64,584
Depreciation, depletion and amortization	394,668		240,232		200,179
Accretion expense	7,534		6,837		6,178
Income tax expense	157,670		154,180		166,750
Results of operations from producing activities	281,573		273,504		294,779
Mark-to-market gain (loss) on derivatives	$ 58,750		$ (37,587)		$ (3)

[1] The years ended December 31, 2012, 2011 and 2010 gross revenues includes a pre-tax non cash mark-to-market gain on derivatives of $58.8 million, a pre-tax non-cash mark-to-market loss on derivatives of $37.6 million and a pre-tax non-cash mark-to-market loss on derivatives of $3,000, respectively.