Industry Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 segment	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information
Number of business segments									2
Segment Reporting Information, Profit (Loss)
Total operating revenues	$ 472,733	$ 320,406	$ 471,495	$ 474,016	$ 414,297	$ 276,429	$ 451,904	$ 398,189	$ 1,738,650	$ 1,540,819	$ 1,373,113
Operating income (loss) from continuing operations	110,630	(4,052)	142,433	102,190	111,609	13,964	215,847	120,494	351,201	461,914	393,699
Depreciation, depletion and amortization expense from continuing operations									497,381	385,453	253,757
Interest expense									69,200	65,542	44,822
Income tax expense (benefit) from continuing operations									105,282	144,534	126,322
Capital expenditures									1,193,514	1,363,080	1,189,436
Identifiable assets	6,622,212				6,175,890				6,622,212	6,175,890	5,237,416
Property, plant and equipment, net	6,003,638				5,541,636				6,003,638	5,541,636	4,620,776
Oil and Gas Operations
Segment Reporting Information, Profit (Loss)
Oil and gas operations									1,205,312	1,089,230	838,160
Operating income (loss) from continuing operations									257,963	369,765	308,561
Depreciation, depletion and amortization expense from continuing operations									453,474	343,183	213,841
Interest expense									53,981	49,958	30,907
Income tax expense (benefit) from continuing operations									71,290	115,090	100,700
Capital expenditures									1,104,745	1,291,211	1,115,452
Identifiable assets	5,379,135				4,975,170				5,379,135	4,975,170	4,046,242
Property, plant and equipment, net	5,116,958				4,697,683				5,116,958	4,697,683	3,806,787
Natural Gas Distribution
Segment Reporting Information, Profit (Loss)
Natural gas distribution									533,338	451,589	534,953
Operating income (loss) from continuing operations									93,768	93,216	86,216
Depreciation, depletion and amortization expense from continuing operations									43,907	42,270	39,916
Interest expense									15,649	16,284	14,740
Income tax expense (benefit) from continuing operations									34,687	30,244	26,670
Capital expenditures									88,769	71,869	73,984
Identifiable assets	1,193,413				1,177,134				1,193,413	1,177,134	1,163,959
Property, plant and equipment, net	885,550				842,685				885,550	842,685	813,471
Eliminations and other
Segment Reporting Information, Profit (Loss)
Operating income (loss) from continuing operations									(530)	(1,067)	(1,078)
Interest expense									(430)	(700)	(825)
Income tax expense (benefit) from continuing operations									(695)	(800)	(1,048)
Identifiable assets	49,664				23,586				49,664	23,586	27,215
Property, plant and equipment, net	$ 1,130				$ 1,268				$ 1,130	$ 1,268	$ 518