Segment Information (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 330,399	$ 333,725	$ 816,763	$ 908,639
Operating income (loss)	106,335	98,293	266,216	290,491
Other income (expense)	(8,797)	(10,805)	(17,134)	(19,951)
Income before income taxes	97,538	87,488	249,082	270,540
Identifiable assets	4,511,394		4,511,394		4,363,560
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues	244,090	234,586	460,882	472,200
Operating income (loss)	105,418	95,456	190,477	212,741
Other income (expense)	(5,822)	(6,555)	(11,269)	(12,426)
Identifiable assets	3,404,189		3,404,189		3,160,601
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	86,309	99,139	355,881	436,439
Operating income (loss)	1,163	3,138	76,222	78,393
Other income (expense)	(3,040)	(3,702)	(5,953)	(6,763)
Identifiable assets	1,070,831		1,070,831		1,166,899
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(246)	(301)	(483)	(643)
Other income (expense)	65	(548)	88	(762)
Identifiable assets	$ 36,374		$ 36,374		$ 36,060