Industry Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 segment	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information
Number of business segments										2
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 288,148	[1]	$ 378,568	$ 330,399	$ 486,364	$ 374,091	$ 295,804	$ 333,725	$ 574,914	$ 1,483,479	$ 1,578,534	$ 1,440,420
Operating income (loss)	31,641	[1]	150,412	106,335	159,881	135,566	67,320	98,293	192,198	448,269	493,377	435,393
Depreciation, depletion and amortization expense										283,997	247,865	235,084
Interest expense										44,822	39,222	39,379
Income tax expense (benefit)										145,701	166,990	143,971
Captial expenditures										1,189,436	811,348	505,208
Identifiable assets	5,237,416					4,363,560				5,237,416	4,363,560	3,803,118
Property, plant and equipment, net	4,620,776					3,719,227				4,620,776	3,719,227	3,144,469
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Operating revenues										948,526	958,762	822,546
Operating income (loss)										363,131	406,729	353,645
Depreciation, depletion and amortization expense										244,081	203,823	184,089
Interest expense										30,907	25,753	25,775
Income tax expense (benefit)										120,079	138,775	117,969
Captial expenditures										1,115,452	717,782	427,399
Identifiable assets	4,046,242					3,160,601				4,046,242	3,160,601	2,654,068
Property, plant and equipment, net	3,806,787					2,936,284				3,806,787	2,936,284	2,422,623
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Operating revenues										534,953	619,772	617,874
Operating income (loss)										86,216	88,383	83,984
Depreciation, depletion and amortization expense										39,916	44,042	50,995
Interest expense										14,740	13,894	13,714
Income tax expense (benefit)										26,670	29,875	27,353
Captial expenditures										73,984	93,566	77,809
Identifiable assets	1,163,959					1,166,899				1,163,959	1,166,899	1,084,666
Property, plant and equipment, net	813,471					782,665				813,471	782,665	721,846
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss)										(1,078)	(1,735)	(2,236)
Interest expense										(825)	(425)	(110)
Income tax expense (benefit)										(1,048)	(1,660)	(1,351)
Identifiable assets	27,215					36,060				27,215	36,060	64,384
Property, plant and equipment, net	$ 518					$ 278				$ 518	$ 278	$ 0

[1] The three months ended December 31, 2011 includes an after-tax non-cash mark-to-market loss on derivatives of $56.6 million, or $0.78 per diluted share.