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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Segment Reporting Information [Line Items] |  |
Revenues | $ 1,042,853 | $ 931,604
Intersegment operating revenues | 0 | 0
Earnings on common stock | 56,491 | 56,344
Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 491,541 | 424,124
Intersegment operating revenues | 18,276 | 18,718
Earnings on common stock | 42,645 | 44,673
Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 551,312 | 507,480
Intersegment operating revenues | 30,346 | 17,498
Earnings on common stock | 14,197 | 11,671
Electric [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 73,647 | 64,654
Intersegment operating revenues | 0 | 0
Earnings on common stock | 11,033 | 9,825
Natural gas distribution [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 374,233 | 331,754
Intersegment operating revenues | 0 | 0
Earnings on common stock | 27,263 | 32,518
Pipeline and energy services [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 43,661 | 27,716
Intersegment operating revenues | 18,276 | 18,718
Earnings on common stock | 4,349 | 2,330
Exploration and production [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 116,669 | 115,363
Intersegment operating revenues | 20,867 | 9,812
Earnings on common stock | 20,939 | 20,284
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 164,423 | 161,977
Intersegment operating revenues | 4,017 | 4,294
Earnings on common stock | (23,574) | (20,582)
Construction services [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 269,892 | 229,806
Intersegment operating revenues | 3,738 | 1,574
Earnings on common stock | 16,568 | 11,664
Other [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 328 | 334
Intersegment operating revenues | 1,724 | 1,818
Earnings on common stock | 264 | 305
Intersegment Eliminations [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 0 | 0
Intersegment operating revenues | (48,622) | (36,216)
Earnings on common stock | $ (351) | $ 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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- 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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