Oil and Gas Operations (Unaudited) (Results of Operations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Gross revenues	$ 1,206,293	[1]	$ 1,090,948	[1]	$ 834,700	[1]
Production (lifting costs)	351,541		278,193		226,361
Exploration expense	27,942		19,356		12,967
Depreciation, depletion and amortization	449,700		339,569		210,532
Accretion expense	6,995		6,339		5,699
Income tax expense	128,773		160,551		134,564
Results of operations from producing activities	241,342		286,940		244,577
Mark-to-market gain (loss) on derivatives	$ (47,832)		$ 58,750		$ (37,587)

[1] The years ended December 31, 2013, 2012 and 2011 gross revenues include a pre-tax non-cash mark-to-market loss on derivatives of $47.8 million, a pre-tax non-cash mark-to-market gain on derivatives of $58.8 million and a pre-tax non-cash mark-to-market loss on derivatives of $37.6 million, respectively.