Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 418,444	$ 486,364
Operating income (loss)	104,170	159,881
Other income (expense)	(13,506)	(8,337)
Income before income taxes	90,664	151,544
Identifiable assets	5,432,772		5,237,416
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues	223,957	216,792
Operating income (loss)	26,005	85,059
Other income (expense)	(10,558)	(5,447)
Identifiable assets	4,249,943		4,046,242
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	194,487	269,572
Operating income (loss)	78,560	75,059
Other income (expense)	(3,004)	(2,913)
Identifiable assets	1,142,781		1,163,959
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(395)	(237)
Other income (expense)	56	23
Identifiable assets	$ 40,048		$ 27,215