Business segment data (Details) (USD $)	3 Months Ended											12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Total external operating revenues	$ 1,081,144,000		$ 1,173,518,000	$ 967,962,000		$ 852,807,000	$ 1,065,749,000		$ 1,152,181,000	$ 930,757,000	$ 901,805,000	$ 4,075,431,000		$ 4,050,492,000		$ 3,909,695,000
Total intersegment operating revenues												0		0		0
Depreciation, depletion and amortization												359,205,000		343,395,000		328,843,000
Interest expense												76,699,000		81,354,000		83,011,000
Income taxes												(31,146,000)		110,274,000		122,530,000
Earnings (loss) on common stock before income (loss) from discontinued operations												(15,006,000)		225,267,000		243,335,000
Income (loss) from discontinued operations, net of tax	8,700,000	[1]	(139,000)	5,106,000	[2]	(100,000)	(13,080,000)	[1]	(126,000)	(168,000)	448,000	13,567,000	[3]	(12,926,000)	[3]	(3,361,000)	[3]
Total earnings (loss) on common stock												(1,439,000)		212,341,000		239,974,000
Net proceeds from sale or disposition of property and other												(57,460,000)		(40,857,000)		(78,761,000)
Total net capital expenditures												933,777,000		481,023,000		495,246,000
Total assets	6,682,491,000						6,556,125,000					6,682,491,000		6,556,125,000		6,303,549,000
Property, plant and equipment	8,107,751,000						7,646,222,000					8,107,751,000		7,646,222,000
Less accumulated depreciation, depletion and amortization	3,608,912,000						3,361,208,000					3,608,912,000		3,361,208,000		3,103,323,000
Net property, plant and equipment	4,498,839,000						4,285,014,000					4,498,839,000		4,285,014,000		4,115,180,000
Additional information [Abstract]
Write-downs of oil and natural gas properties	231,700,000		160,100,000									391,800,000		0		0
Noncash write-down of natural gas and oil properties, after tax	145,900,000		100,900,000									246,800,000
Natural gas gathering arbitration charge, after tax																16,500,000
Reversal of an arbitration charge after tax				15,000,000								15,000,000
Net noncash transactions included in capital expenditures												33,700,000		24,000,000		17,500,000
Regulated:
Segment Reporting Information [Line Items]
Total external operating revenues												1,131,626,000		1,343,714,000		1,359,028,000
Nonregulated:
Segment Reporting Information [Line Items]
Total external operating revenues												2,943,805,000		2,706,778,000		2,550,667,000
Electric:
Segment Reporting Information [Line Items]
Total external operating revenues												236,895,000		225,468,000		211,544,000
Total intersegment operating revenues												0		0		0
Depreciation, depletion and amortization												32,509,000		32,177,000		27,274,000
Interest expense												12,421,000		13,745,000		12,216,000
Income taxes												8,975,000		7,242,000		11,187,000
Earnings (loss) on common stock before income (loss) from discontinued operations												30,634,000		29,258,000		28,908,000
Capital expenditures												112,035,000		52,072,000		85,787,000
Total assets	760,324,000	[4]					672,940,000	[4]				760,324,000	[4]	672,940,000	[4]	643,636,000	[4]
Property, plant and equipment	1,150,584,000	[4]					1,068,524,000	[4]				1,150,584,000	[4]	1,068,524,000	[4]	1,027,034,000	[4]
Natural gas distribution:
Segment Reporting Information [Line Items]
Total external operating revenues												754,848,000		907,400,000		892,708,000
Total intersegment operating revenues												0		0		0
Depreciation, depletion and amortization												45,731,000		44,641,000		43,044,000
Interest expense												28,726,000		29,444,000		28,996,000
Income taxes												12,005,000		16,931,000		12,171,000
Earnings (loss) on common stock before income (loss) from discontinued operations												29,409,000		38,398,000		36,944,000
Capital expenditures												130,178,000		70,624,000		75,365,000
Total assets	1,703,459,000	[4]					1,679,091,000	[4]				1,703,459,000	[4]	1,679,091,000	[4]	1,632,012,000	[4]
Property, plant and equipment	1,689,950,000	[4]					1,568,866,000	[4]				1,689,950,000	[4]	1,568,866,000	[4]	1,508,845,000	[4]
Pipeline and energy services:
Segment Reporting Information [Line Items]
Total external operating revenues												139,883,000		210,846,000		254,776,000
Total intersegment operating revenues												53,274,000		67,497,000		75,033,000
Depreciation, depletion and amortization												27,684,000		25,502,000		26,001,000
Interest expense												7,742,000		10,516,000		9,064,000
Income taxes												15,291,000		12,912,000		13,933,000
Earnings (loss) on common stock before income (loss) from discontinued operations												26,588,000		23,082,000		23,208,000
Capital expenditures												133,787,000		45,556,000		14,255,000
Total assets	622,470,000						526,797,000					622,470,000		526,797,000		523,075,000
Property, plant and equipment	816,533,000						719,291,000					816,533,000		719,291,000		683,807,000
Exploration and production:
Segment Reporting Information [Line Items]
Total external operating revenues												412,651,000		359,873,000		318,570,000
Total intersegment operating revenues												35,966,000		93,713,000		115,784,000
Depreciation, depletion and amortization												160,681,000		142,645,000		130,455,000
Interest expense												9,018,000		7,445,000		8,580,000
Income taxes												(108,264,000)		46,298,000		49,034,000
Earnings (loss) on common stock before income (loss) from discontinued operations												(177,283,000)		80,282,000		85,638,000
Capital expenditures												554,528,000		272,855,000		355,845,000
Total assets	1,539,017,000						1,481,556,000					1,539,017,000		1,481,556,000		1,342,808,000
Property, plant and equipment	2,764,560,000						2,615,146,000					2,764,560,000		2,615,146,000		2,356,938,000
Construction materials and contracting:
Segment Reporting Information [Line Items]
Total external operating revenues												1,597,257,000		1,509,538,000		1,445,148,000
Total intersegment operating revenues												20,168,000		472,000		0
Depreciation, depletion and amortization												79,527,000		85,459,000		88,331,000
Interest expense												15,211,000		16,241,000		19,859,000
Income taxes												14,099,000		11,227,000		13,822,000
Earnings (loss) on common stock before income (loss) from discontinued operations												32,420,000		26,430,000		29,609,000
Capital expenditures												45,083,000		52,303,000		25,724,000
Total assets	1,371,252,000						1,374,026,000					1,371,252,000		1,374,026,000		1,382,836,000
Property, plant and equipment	1,504,981,000						1,499,852,000					1,504,981,000		1,499,852,000		1,486,375,000
Construction services:
Segment Reporting Information [Line Items]
Total external operating revenues												932,013,000		834,918,000		786,802,000
Total intersegment operating revenues												6,545,000		19,471,000		2,298,000
Depreciation, depletion and amortization												11,063,000		11,399,000		12,147,000
Interest expense												4,435,000		4,473,000		4,411,000
Income taxes												24,128,000		13,426,000		11,456,000
Earnings (loss) on common stock before income (loss) from discontinued operations												38,429,000		21,627,000		17,982,000
Capital expenditures												14,835,000		9,711,000		14,849,000
Total assets	429,547,000						418,519,000					429,547,000		418,519,000		387,627,000
Property, plant and equipment	130,624,000						124,796,000					130,624,000		124,796,000		122,940,000
Other [Member]
Segment Reporting Information [Line Items]
Total external operating revenues												1,884,000		2,449,000		147,000
Total intersegment operating revenues												8,486,000		8,997,000		7,580,000
Depreciation, depletion and amortization												2,010,000		1,572,000		1,591,000
Interest expense												13,000		0		47,000
Income taxes												2,620,000		2,238,000		10,927,000
Earnings (loss) on common stock before income (loss) from discontinued operations												4,797,000		6,190,000		21,046,000
Capital expenditures												791,000		18,759,000		2,182,000
Total assets	256,422,000	[5]					403,196,000	[5]				256,422,000	[5]	403,196,000	[5]	391,555,000	[5]
Property, plant and equipment	50,519,000						49,747,000					50,519,000		49,747,000		32,564,000
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
Total intersegment operating revenues												(124,439,000)		(190,150,000)		(200,695,000)
Interest expense												$ (867,000)		$ (510,000)		$ (162,000)

[1] 2012 reflects a $145.9 million after-tax noncash write-down of oil and natural gas properties and the reversal of an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract, which was partially offset by the reversal of estimated insurance recoveries, as previously discussed. 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Notes 1 and 19, respectively.

[2] 2012 reflects a net benefit of $15.0 million (after tax) related to natural gas gathering operations litigation and a net benefit largely related to estimated insurance recoveries related to the guarantee of a construction contract. For more information, see Note 19.

[3] Reflected in the Other category.

[4] Includes allocations of common utility property.

[5] 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).