Business segment data (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
External operating revenues	$ 1,152,181,000	$ 1,125,923,000	$ 2,984,743,000	$ 2,867,144,000
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	63,803,000	60,838,000	151,505,000	151,206,000
Income (loss) on discontinued operations	(126,000)	0	154,000	0
Contract Dispute Charge To Operation And Maintenance Expense After Tax		16,500,000		16,500,000
Regulated Operation [Member]
Segment Reporting Information [Line Items]
External operating revenues	212,848,000	223,602,000	964,866,000	956,025,000
Inter-segment operating revenues	10,591,000	11,940,000	47,836,000	53,168,000
Earnings on common stock	2,350,000	(6,165,000)	56,790,000	46,416,000
Regulated Operation [Member] | Electric [Member]
Segment Reporting Information [Line Items]
External operating revenues	61,949,000	59,966,000	169,780,000	155,345,000
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	8,312,000	11,259,000	21,642,000	22,091,000
Regulated Operation [Member] | Natural gas distribution [Member]
Segment Reporting Information [Line Items]
External operating revenues	92,440,000	94,336,000	627,450,000	603,499,000
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	(11,183,000)	(10,054,000)	18,235,000	13,362,000
Regulated Operation [Member] | Pipeline and energy services [Member]
Segment Reporting Information [Line Items]
External operating revenues	58,459,000	69,300,000	167,636,000	197,181,000
Inter-segment operating revenues	10,591,000	11,940,000	47,836,000	53,168,000
Earnings on common stock	5,221,000	(7,370,000)	16,913,000	10,963,000
Unregulated Operation [Member]
Segment Reporting Information [Line Items]
External operating revenues	939,333,000	902,321,000	2,019,877,000	1,911,119,000
Inter-segment operating revenues	29,325,000	30,139,000	91,443,000	96,950,000
Earnings on common stock	61,453,000	67,003,000	94,715,000	104,790,000
Unregulated Operation [Member] | Construction services [Member]
Segment Reporting Information [Line Items]
External operating revenues	222,822,000	210,362,000	617,699,000	551,608,000
Inter-segment operating revenues	3,344,000	137,000	9,940,000	170,000
Earnings on common stock	5,044,000	5,990,000	15,815,000	9,041,000
Unregulated Operation [Member] | Natural gas and oil production [Member]
Segment Reporting Information [Line Items]
External operating revenues	96,803,000	79,276,000	262,604,000	235,342,000
Inter-segment operating revenues	23,956,000	27,739,000	74,889,000	90,066,000
Earnings on common stock	22,497,000	18,717,000	60,093,000	64,963,000
Unregulated Operation [Member] | Construction materials and contracting [Member]
Segment Reporting Information [Line Items]
External operating revenues	619,134,000	612,654,000	1,138,280,000	1,124,086,000
Inter-segment operating revenues	0	0	0	0
Earnings on common stock	33,103,000	40,257,000	16,680,000	25,779,000
Unregulated Operation [Member] | Other [Member]
Segment Reporting Information [Line Items]
External operating revenues	574,000	29,000	1,294,000	83,000
Inter-segment operating revenues	2,025,000	2,263,000	6,614,000	6,714,000
Earnings on common stock	809,000	2,039,000	2,127,000	5,007,000
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External operating revenues	0	0	0	0
Inter-segment operating revenues	(39,916,000)	(42,079,000)	(139,279,000)	(150,118,000)
Earnings on common stock	$ 0	$ 0	$ 0	$ 0