Business segment data (Details) (USD $)	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		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Total external operating revenues	$ 1,065,749,000		$ 1,152,181,000	$ 930,757,000	$ 901,805,000	$ 1,042,551,000		$ 1,125,923,000	$ 906,444,000	$ 834,777,000	$ 4,050,492,000		$ 3,909,695,000		$ 4,176,501,000
Total intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											343,395,000		328,843,000		330,542,000
Interest expense											81,354,000		83,011,000		84,099,000
Income taxes											110,274,000		122,530,000		(96,092,000)
Earnings (loss) on common stock before income (loss) from discontinued operations											225,267,000		243,335,000		(123,959,000)
Income (loss) from discontinued operations, net of tax	(13,080,000)	[1]	(126,000)	(168,000)	448,000	(3,361,000)		0	0	0	(12,926,000)	[2]	(3,361,000)	[2]	0	[2]
Earnings (loss) on common stock											212,341,000		239,974,000		(123,959,000)
Net proceeds from sale or disposition of property and other											40,857,000		78,761,000		26,679,000
Net capital expenditures											481,023,000		495,246,000		428,012,000
Total assets	6,556,125,000					6,303,549,000					6,556,125,000		6,303,549,000		5,990,952,000
Property, plant and equipment	7,646,222,000					7,218,503,000					7,646,222,000		7,218,503,000
Less accumulated depreciation, depletion and amortization	3,361,208,000					3,103,323,000					3,361,208,000		3,103,323,000		2,872,465,000
Net property, plant and equipment	4,285,014,000					4,115,180,000					4,285,014,000		4,115,180,000		3,894,117,000
Additional information [Abstract]
Noncash write-down of natural gas and oil properties											0		0		620,000,000
Noncash write-down of natural gas and oil properties, after tax															384,400,000
Natural gas gathering arbitration charge, after tax								16,500,000					16,500,000
Net noncash transactions included in capital expenditures											24,000,000		17,500,000
Regulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											1,343,714,000		1,359,028,000		1,504,269,000
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											2,706,778,000		2,550,667,000		2,672,232,000
Electric [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											225,468,000		211,544,000		196,171,000
Total intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											32,177,000		27,274,000		24,637,000
Interest expense											13,745,000		12,216,000		9,577,000
Income taxes											7,242,000		11,187,000		8,205,000
Earnings (loss) on common stock before income (loss) from discontinued operations											29,258,000		28,908,000		24,099,000
Net capital expenditures											52,072,000		85,787,000		115,240,000
Total assets	672,940,000	[3]				643,636,000	[3]				672,940,000	[3]	643,636,000	[3]	569,666,000	[3]
Property, plant and equipment	1,068,524,000	[3]				1,027,034,000	[3]				1,068,524,000	[3]	1,027,034,000	[3]	941,791,000	[3]
Natural gas distribution [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											907,400,000		892,708,000		1,072,776,000
Total intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											44,641,000		43,044,000		42,723,000
Interest expense											29,444,000		28,996,000		30,656,000
Income taxes											16,931,000		12,171,000		16,331,000
Earnings (loss) on common stock before income (loss) from discontinued operations											38,398,000		36,944,000		30,796,000
Net capital expenditures											70,624,000		75,365,000		43,820,000
Total assets	1,679,091,000	[3]				1,632,012,000	[3]				1,679,091,000	[3]	1,632,012,000	[3]	1,588,144,000	[3]
Property, plant and equipment	1,568,866,000	[3]				1,508,845,000	[3]				1,568,866,000	[3]	1,508,845,000	[3]	1,456,208,000	[3]
Pipeline and energy services [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											210,846,000		254,776,000		235,322,000
Total intersegment operating revenues											67,497,000		75,033,000		72,505,000
Depreciation, depletion and amortization											25,502,000		26,001,000		25,581,000
Interest expense											10,516,000		9,064,000		8,896,000
Income taxes											12,912,000		13,933,000		22,982,000
Earnings (loss) on common stock before income (loss) from discontinued operations											23,082,000		23,208,000		37,845,000
Net capital expenditures											45,556,000		14,255,000		70,168,000
Total assets	526,797,000					523,075,000					526,797,000		523,075,000		538,230,000
Property, plant and equipment	719,291,000					683,807,000					719,291,000		683,807,000		675,199,000
Exploration and production [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											359,873,000		318,570,000		338,425,000
Total intersegment operating revenues											93,713,000		115,784,000		101,230,000
Depreciation, depletion and amortization											142,645,000		130,455,000		129,922,000
Interest expense											7,445,000		8,580,000		10,621,000
Income taxes											46,298,000		49,034,000		(187,000,000)
Earnings (loss) on common stock before income (loss) from discontinued operations											80,282,000		85,638,000		(296,730,000)
Net capital expenditures											272,855,000		355,845,000		183,140,000
Total assets	1,481,556,000					1,342,808,000					1,481,556,000		1,342,808,000		1,137,628,000
Property, plant and equipment	2,615,146,000					2,356,938,000					2,615,146,000		2,356,938,000		2,028,794,000
Construction materials and contracting [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											1,509,538,000		1,445,148,000		1,515,122,000
Total intersegment operating revenues											472,000		0		0
Depreciation, depletion and amortization											85,459,000		88,331,000		93,615,000
Interest expense											16,241,000		19,859,000		20,495,000
Income taxes											11,227,000		13,822,000		25,940,000
Earnings (loss) on common stock before income (loss) from discontinued operations											26,430,000		29,609,000		47,085,000
Net capital expenditures											52,303,000		25,724,000		26,313,000
Total assets	1,374,026,000					1,382,836,000					1,374,026,000		1,382,836,000		1,449,469,000
Property, plant and equipment	1,499,852,000					1,486,375,000					1,499,852,000		1,486,375,000		1,514,989,000
Construction services [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											834,918,000		786,802,000		818,685,000
Total intersegment operating revenues											19,471,000		2,298,000		379,000
Depreciation, depletion and amortization											11,399,000		12,147,000		12,760,000
Interest expense											4,473,000		4,411,000		4,490,000
Income taxes											13,426,000		11,456,000		15,189,000
Earnings (loss) on common stock before income (loss) from discontinued operations											21,627,000		17,982,000		25,589,000
Net capital expenditures											9,711,000		14,849,000		12,814,000
Total assets	418,519,000					387,627,000					418,519,000		387,627,000		328,895,000
Property, plant and equipment	124,796,000					122,940,000					124,796,000		122,940,000		116,236,000
Other [Member]
Segment Reporting Information [Line Items]
Total external operating revenues											2,449,000		147,000		0
Total intersegment operating revenues											8,997,000		7,580,000		9,487,000
Depreciation, depletion and amortization											1,572,000		1,591,000		1,304,000
Interest expense											0		47,000		43,000
Income taxes											2,238,000		10,927,000		2,261,000
Earnings (loss) on common stock before income (loss) from discontinued operations											6,190,000		21,046,000		7,357,000
Net capital expenditures											18,759,000		2,182,000		3,196,000
Total assets	403,196,000	[4]				391,555,000	[4]				403,196,000	[4]	391,555,000	[4]	378,920,000	[4]
Property, plant and equipment	49,747,000					32,564,000					49,747,000		32,564,000		33,365,000
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Total intersegment operating revenues											(190,150,000)		(200,695,000)		(183,601,000)
Interest expense											$ (510,000)		$ (162,000)		$ (679,000)

[1] 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Note 19.

[2] Reflected in the Other category.

[3] Includes allocations of common utility property.

[4] Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets).