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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,060,595 | $ 967,962 | $ 1,992,198 | $ 1,820,769
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 46,341 | 53,942 | 102,685 | 89,561
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 227,442 | 204,455 | 651,565 | 599,533
Intersegment operating revenues | 7,999 | 8,937 | 26,717 | 29,347
Earnings on common stock | (7,878) | 13,859 | 36,795 | 49,686
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 833,153 | 763,507 | 1,340,633 | 1,221,236
Intersegment operating revenues | 29,693 | 11,055 | 47,191 | 22,885
Earnings on common stock | 56,275 | 40,083 | 67,946 | 39,875
Electric [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 56,981 | 52,955 | 121,635 | 110,918
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 4,410 | 4,419 | 14,235 | 11,978
Natural gas distribution [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 127,584 | 116,844 | 459,337 | 424,733
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | (5,893) | (6,411) | 26,624 | 19,097
Pipeline and energy services [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 42,877 | 34,656 | 70,593 | 63,882
Intersegment operating revenues | 7,999 | 8,937 | 26,717 | 29,347
Earnings on common stock | (6,395) | 15,851 | (4,064) | 18,611
Exploration and production [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 137,053 | 100,232 | 252,415 | 188,727
Intersegment operating revenues | 12,556 | 5,711 | 22,369 | 17,038
Earnings on common stock | 32,995 | 17,957 | 53,279 | 30,887
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 418,345 | 438,963 | 580,323 | 588,232
Intersegment operating revenues | 12,958 | 3,097 | 17,251 | 3,248
Earnings on common stock | 10,025 | 7,791 | (10,557) | (17,141)
Construction services [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 277,259 | 223,858 | 507,065 | 442,010
Intersegment operating revenues | 2,340 | 219 | 3,914 | 244
Earnings on common stock | 12,915 | 8,684 | 24,579 | 20,087
Other [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 496 | 454 | 830 | 2,267
Intersegment operating revenues | 1,839 | 2,028 | 3,657 | 2,355
Earnings on common stock | 340 | 5,651 | 645 | 6,042
Intersegment Eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Intersegment operating revenues | (37,692) | (19,992) | (73,908) | (52,232)
Earnings on common stock | $ (2,056) | $ 0 | $ (2,056) | $ 0

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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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