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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

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