Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 470,355	$ 330,399	$ 888,799	$ 816,763
Operating income (loss)	220,598	106,335	324,768	266,216
Other income (expense)	(15,758)	(8,797)	(29,264)	(17,134)
Income before income taxes	204,840	97,538	295,504	249,082
Identifiable assets	5,779,540		5,779,540		5,237,416
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues	399,468	244,090	623,425	460,882
Operating income (loss)	216,406	105,418	242,411	190,477
Other income (expense)	(12,004)	(5,822)	(22,562)	(11,269)
Identifiable assets	4,596,172		4,596,172		4,046,242
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	70,887	86,309	265,374	355,881
Operating income (loss)	4,448	1,163	83,008	76,222
Other income (expense)	(3,784)	(3,040)	(6,788)	(5,953)
Identifiable assets	1,108,228		1,108,228		1,163,959
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(256)	(246)	(651)	(483)
Other income (expense)	30	65	86	88
Identifiable assets	$ 75,140		$ 75,140		$ 27,215