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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,370,455 | $ 1,285,782 | $ 3,507,354 | $ 3,277,980
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 103,038 | 84,285 | 213,464 | 186,970
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 211,536 | 192,103 | 957,769 | 843,670
Intersegment operating revenues | 4,334 | 4,906 | 30,497 | 31,623
Earnings on common stock | 1,970 | 5,523 | 53,732 | 42,319
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 1,158,919 | 1,093,679 | 2,549,585 | 2,434,310
Intersegment operating revenues | 33,473 | 23,092 | 91,214 | 70,283
Earnings on common stock | 102,590 | 79,964 | 162,437 | 147,910
Electric [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 68,936 | 68,314 | 207,732 | 189,949
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 9,162 | 11,417 | 28,018 | 25,652
Natural gas distribution [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 96,185 | 77,417 | 616,496 | 536,756
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | (12,252) | (11,204) | 10,516 | 15,420
Pipeline and energy services [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 46,415 | 46,372 | 133,541 | 116,965
Intersegment operating revenues | 4,334 | 4,906 | 30,497 | 31,623
Earnings on common stock | 5,060 | 5,310 | 15,198 | 1,247
Exploration and production [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 147,677 | 119,234 | 393,653 | 371,648
Intersegment operating revenues | 8,130 | 10,714 | 39,269 | 33,083
Earnings on common stock | 34,750 | 17,434 | 74,869 | 70,713
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 740,496 | 706,982 | 1,339,371 | 1,287,305
Intersegment operating revenues | 6,322 | 7,422 | 18,445 | 24,673
Earnings on common stock | 55,218 | 49,159 | 42,199 | 38,602
Construction services [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 270,313 | 267,038 | 815,313 | 774,103
Intersegment operating revenues | 16,420 | 3,097 | 27,431 | 7,011
Earnings on common stock | 9,876 | 12,154 | 40,751 | 36,733
Other [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 433 | 425 | 1,248 | 1,254
Intersegment operating revenues | 2,601 | 1,859 | 6,069 | 5,516
Earnings on common stock | 2,746 | 1,217 | 4,618 | 1,862
Intersegment eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Intersegment operating revenues | (37,807) | (27,998) | (121,711) | (101,906)
Earnings on common stock | $ (1,522) | $ (1,202) | $ (2,705) | $ (3,259)

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- Definition Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256 + Details
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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5 + Details
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