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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2012 | Mar. 31, 2011
Segment Reporting Information [Line Items] |  |
Total external operating revenues | $ 852,807 | $ 901,805
Total intersegment operating revenues | 0 | 0
Earnings on common stock | 35,619 | 42,806
Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 395,081 | 477,481
Total intersegment operating revenues | 20,409 | 24,741
Earnings on common stock | 35,827 | 42,960
Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 457,726 | 424,324
Total intersegment operating revenues | 11,831 | 29,046
Earnings on common stock | (208) | (154)
Electric [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 57,963 | 57,845
Total intersegment operating revenues | 0 | 0
Earnings on common stock | 7,559 | 8,524
Natural gas distribution [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 307,891 | 370,385
Total intersegment operating revenues | 0 | 0
Earnings on common stock | 25,508 | 27,516
Pipeline and energy services [Member] | Regulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 29,227 | 49,251
Total intersegment operating revenues | 20,409 | 24,741
Earnings on common stock | 2,760 | 6,920
Exploration and production [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 88,494 | 78,410
Total intersegment operating revenues | 11,328 | 25,541
Earnings on common stock | 12,930 | 16,269
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 149,268 | 143,533
Total intersegment operating revenues | 151 | 0
Earnings on common stock | (24,932) | (21,402)
Construction services [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 218,151 | 202,180
Total intersegment operating revenues | 25 | 1,217
Earnings on common stock | 11,403 | 4,632
Other [Member] | Unregulated Operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 1,813 | 201
Total intersegment operating revenues | 327 | 2,288
Earnings on common stock | 391 | 347
Intersegment Elimination [Member] |  |
Segment Reporting Information [Line Items] |  |
Total external operating revenues | 0 | 0
Total intersegment operating revenues | (32,240) | (53,787)
Earnings on common stock | $ 0 | $ 0

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- Definition Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1 Reference 2: 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=6880815&loc=d3e20235-122688 + Details
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- Definition Amount of revenue from transactions with other operating segments of the same entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 131 -Paragraph 27 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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