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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,285,782 | $ 1,173,518 | $ 3,277,980 | $ 2,994,288
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 84,285 | (29,842) | 186,970 | 59,719
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 192,103 | 184,863 | 843,670 | 784,399
Intersegment operating revenues | 4,906 | 7,046 | 31,623 | 36,393
Earnings on common stock | 5,523 | 5,491 | 42,319 | 55,175
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 1,093,679 | 988,655 | 2,434,310 | 2,209,889
Intersegment operating revenues | 23,092 | 19,366 | 70,283 | 42,251
Earnings on common stock | 79,964 | (35,333) | 147,910 | 4,544
Electric [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 68,314 | 63,492 | 189,949 | 174,410
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 11,417 | 11,000 | 25,652 | 22,977
Natural gas distribution [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 77,417 | 80,069 | 536,756 | 504,805
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | (11,204) | (8,782) | 15,420 | 10,314
Pipeline and energy services [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 46,372 | 41,302 | 116,965 | 105,184
Intersegment operating revenues | 4,906 | 7,046 | 31,623 | 36,393
Earnings on common stock | 5,310 | 3,273 | 1,247 | 21,884
Exploration and production [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 119,234 | 100,380 | 371,648 | 289,106
Intersegment operating revenues | 10,714 | 8,076 | 33,083 | 25,114
Earnings on common stock | 17,434 | (87,748) | 70,713 | (56,860)
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 706,982 | 641,500 | 1,287,305 | 1,229,731
Intersegment operating revenues | 7,422 | 8,508 | 24,673 | 11,756
Earnings on common stock | 49,159 | 41,889 | 38,602 | 24,748
Construction services [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 267,038 | 246,358 | 774,103 | 688,368
Intersegment operating revenues | 3,097 | 834 | 7,011 | 1,078
Earnings on common stock | 12,154 | 9,863 | 36,733 | 29,951
Other [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 425 | 417 | 1,254 | 2,684
Intersegment operating revenues | 1,859 | 1,948 | 5,516 | 4,303
Earnings on common stock | 1,217 | 663 | 1,862 | 6,705
Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Intersegment operating revenues | (27,998) | (26,412) | (101,906) | (78,644)
Earnings on common stock | $ (1,202) | $ 0 | $ (3,259) | $ 0

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