Business segment data (Details) (USD $)	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		Dec. 31, 2012		Dec. 31, 2011
Business segment data [Line Items]
External operating revenues	$ 1,184,423,000		$ 1,285,782,000	$ 1,060,595,000	$ 931,604,000	$ 1,081,144,000		$ 1,173,518,000	$ 967,962,000	$ 852,807,000	$ 4,462,404,000		$ 4,075,431,000		$ 4,050,492,000
Intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											386,856,000		359,205,000		343,395,000
Interest expense											83,917,000		76,699,000		81,354,000
Income taxes											136,736,000		(31,146,000)		110,274,000
Earnings (loss) on common stock before income (loss) from discontinued operations											278,560,000		(15,006,000)		225,267,000
Income (loss) from discontinued operations, net of tax	(58,000)		(118,000)	(59,000)	(77,000)	8,700,000		(139,000)	5,106,000	(100,000)	(312,000)	[1]	13,567,000	[1]	(12,926,000)	[1]
Total earnings (loss) on common stock											278,248,000		(1,439,000)		212,341,000
Net proceeds from sale or disposition of property and other											(112,131,000)		(57,460,000)		(40,857,000)
Total net capital expenditures											728,070,000		933,777,000		481,023,000
Total assets	7,061,332,000					6,682,491,000					7,061,332,000		6,682,491,000		6,556,125,000
Property, plant and equipment	8,803,866,000					8,107,751,000					8,803,866,000		8,107,751,000
Less accumulated depreciation, depletion and amortization	3,872,487,000					3,608,912,000					3,872,487,000		3,608,912,000		3,361,208,000
Net property, plant and equipment	4,931,379,000					4,498,839,000					4,931,379,000		4,498,839,000		4,285,014,000
Additional information [Abstract]
Write-downs of oil and natural gas properties						231,700,000		160,100,000			0		391,800,000		0
Noncash write-down of natural gas and oil properties, after tax						145,900,000		100,900,000					246,800,000
Asset impairment charges				9,000,000					1,700,000		9,000,000		1,700,000
Reversal of an arbitration charge after tax	1,500,000					13,000,000			15,000,000		1,500,000		15,000,000
Net noncash inclusion of acct pay and NCI exclusion of capital expenditures											(56,800,000)		33,700,000		24,000,000
Regulated:
Business segment data [Line Items]
External operating revenues											1,264,574,000		1,131,626,000		1,343,714,000
Nonregulated:
Business segment data [Line Items]
External operating revenues											3,197,830,000		2,943,805,000		2,706,778,000
Electric:
Business segment data [Line Items]
External operating revenues											257,260,000		236,895,000		225,468,000
Intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											32,789,000		32,509,000		32,177,000
Interest expense											12,590,000		12,421,000		13,745,000
Income taxes											9,683,000		8,975,000		7,242,000
Earnings (loss) on common stock before income (loss) from discontinued operations											34,837,000		30,634,000		29,258,000
Capital expenditures											168,557,000		112,035,000		52,072,000
Total assets	884,283,000	[2]				760,324,000	[2]				884,283,000	[2]	760,324,000	[2]	672,940,000	[2]
Property, plant and equipment	1,315,822,000	[2]				1,150,584,000	[2]				1,315,822,000	[2]	1,150,584,000	[2]	1,068,524,000	[2]
Natural gas distribution:
Business segment data [Line Items]
External operating revenues											851,945,000		754,848,000		907,400,000
Intersegment operating revenues											0		0		0
Depreciation, depletion and amortization											50,031,000		45,731,000		44,641,000
Interest expense											25,123,000		28,726,000		29,444,000
Income taxes											16,633,000		12,005,000		16,931,000
Earnings (loss) on common stock before income (loss) from discontinued operations											37,656,000		29,409,000		38,398,000
Capital expenditures											101,279,000		130,178,000		70,624,000
Total assets	1,786,068,000	[2]				1,703,459,000	[2]				1,786,068,000	[2]	1,703,459,000	[2]	1,679,091,000	[2]
Property, plant and equipment	1,776,901,000	[2]				1,689,950,000	[2]				1,776,901,000	[2]	1,689,950,000	[2]	1,568,866,000	[2]
Pipeline and energy services:
Business segment data [Line Items]
External operating revenues											155,369,000		139,883,000		210,846,000
Intersegment operating revenues											46,699,000		53,274,000		67,497,000
Depreciation, depletion and amortization											29,119,000		27,684,000		25,502,000
Interest expense											10,330,000		7,742,000		10,516,000
Income taxes											3,390,000		15,291,000		12,912,000
Earnings (loss) on common stock before income (loss) from discontinued operations											7,629,000		26,588,000		23,082,000
Capital expenditures											127,092,000		133,787,000		45,556,000
Total assets	798,701,000					622,470,000					798,701,000		622,470,000		526,797,000
Property, plant and equipment	962,172,000					816,533,000					962,172,000		816,533,000		719,291,000
Exploration and production:
Business segment data [Line Items]
External operating revenues											490,924,000		412,651,000		359,873,000
Intersegment operating revenues											45,099,000		35,966,000		93,713,000
Depreciation, depletion and amortization											186,458,000		160,681,000		142,645,000
Interest expense											14,315,000		9,018,000		7,445,000
Income taxes											53,197,000		(108,264,000)		46,298,000
Earnings (loss) on common stock before income (loss) from discontinued operations											94,450,000		(177,283,000)		80,282,000
Capital expenditures											391,315,000		554,528,000		272,855,000
Total assets	1,616,131,000					1,539,017,000					1,616,131,000		1,539,017,000		1,481,556,000
Property, plant and equipment	3,060,848,000					2,764,560,000					3,060,848,000		2,764,560,000		2,615,146,000
Construction materials and contracting:
Business segment data [Line Items]
External operating revenues											1,675,444,000		1,597,257,000		1,509,538,000
Intersegment operating revenues											36,693,000		20,168,000		472,000
Depreciation, depletion and amortization											74,470,000		79,527,000		85,459,000
Interest expense											17,394,000		15,211,000		16,241,000
Income taxes											24,765,000		14,099,000		11,227,000
Earnings (loss) on common stock before income (loss) from discontinued operations											50,946,000		32,420,000		26,430,000
Capital expenditures											34,607,000		45,083,000		52,303,000
Total assets	1,305,808,000					1,371,252,000					1,305,808,000		1,371,252,000		1,374,026,000
Property, plant and equipment	1,510,355,000					1,504,981,000					1,510,355,000		1,504,981,000		1,499,852,000
Construction services:
Business segment data [Line Items]
External operating revenues											1,029,909,000		932,013,000		834,918,000
Intersegment operating revenues											9,930,000		6,545,000		19,471,000
Depreciation, depletion and amortization											11,939,000		11,063,000		11,399,000
Interest expense											4,306,000		4,435,000		4,473,000
Income taxes											29,504,000		24,128,000		13,426,000
Earnings (loss) on common stock before income (loss) from discontinued operations											52,213,000		38,429,000		21,627,000
Capital expenditures											15,102,000		14,835,000		9,711,000
Total assets	450,614,000					429,547,000					450,614,000		429,547,000		418,519,000
Property, plant and equipment	134,948,000					130,624,000					134,948,000		130,624,000		124,796,000
Other
Business segment data [Line Items]
External operating revenues											1,553,000		1,884,000		2,449,000
Intersegment operating revenues											8,067,000		8,486,000		8,997,000
Depreciation, depletion and amortization											2,050,000		2,010,000		1,572,000
Interest expense											15,000		13,000		0
Income taxes											2,433,000		2,620,000		2,238,000
Earnings (loss) on common stock before income (loss) from discontinued operations											5,136,000		4,797,000		6,190,000
Capital expenditures											2,249,000		791,000		18,759,000
Total assets	219,727,000	[3]				256,422,000	[3]				219,727,000	[3]	256,422,000	[3]	403,196,000	[3]
Property, plant and equipment	49,997,000					50,519,000					49,997,000		50,519,000		49,747,000
Eliminations
Business segment data [Line Items]
Intersegment operating revenues											(146,488,000)		(124,439,000)		(190,150,000)
Interest expense											(156,000)		(867,000)		(510,000)
Income taxes											(2,869,000)		0		0
Earnings (loss) on common stock before income (loss) from discontinued operations											(4,307,000)		0		0
Property, plant and equipment	$ (7,177,000)					$ 0					$ (7,177,000)		$ 0		$ 0

[1] Reflected in the Other category.

[2] Includes allocations of common utility property.

[3] 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).