Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 295,324	$ 378,568	$ 1,184,123	$ 1,195,331
Operating income (loss)	19,458	150,412	344,226	416,628
Other income (expense)	(15,759)	(13,431)	(45,023)	(30,565)
Income before income taxes	3,699	136,981	299,203	386,063
Identifiable assets	5,919,829		5,919,829		5,237,416
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues	233,515	318,952	856,940	779,834
Operating income (loss)	32,407	161,331	274,818	351,808
Other income (expense)	(12,633)	(9,363)	(35,195)	(20,632)
Identifiable assets	4,734,269		4,734,269		4,046,242
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	61,809	59,616	327,183	415,497
Operating income (loss)	(12,743)	(10,681)	70,265	65,541
Other income (expense)	(3,274)	(4,162)	(10,062)	(10,115)
Identifiable assets	1,120,161		1,120,161		1,163,959
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(206)	(238)	(857)	(721)
Other income (expense)	148	94	234	182
Identifiable assets	$ 65,399		$ 65,399		$ 27,215