Segment Information (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 378,568	$ 295,804	$ 1,195,331	$ 1,204,443
Operating income (loss)	150,412	67,320	416,628	357,811
Other income (expense)	(13,431)	(7,430)	(30,565)	(27,381)
Income before income taxes	136,981	59,890	386,063	330,430
Identifiable assets	4,875,442		4,875,442		4,363,560
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues	318,952	234,111	779,834	706,311
Operating income (loss)	161,331	76,644	351,808	289,385
Other income (expense)	(9,363)	(4,848)	(20,632)	(17,274)
Identifiable assets	3,752,437		3,752,437		3,160,601
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	59,616	61,693	415,497	498,132
Operating income (loss)	(10,681)	(9,015)	65,541	69,378
Other income (expense)	(4,162)	(2,119)	(10,115)	(8,882)
Identifiable assets	1,107,188		1,107,188		1,166,899
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(238)	(309)	(721)	(952)
Other income (expense)	94	(463)	182	(1,225)
Identifiable assets	$ 15,817		$ 15,817		$ 36,060