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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | $ 1,173,518 | $ 1,152,181 | $ 2,994,288 | $ 2,984,743
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | (29,842) | 63,803 | 59,719 | 151,505
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 184,863 | 212,848 | 784,399 | 964,866
Total intersegment operating revenues | 7,046 | 10,591 | 36,393 | 47,836
Earnings (loss) on common stock | 5,491 | 2,350 | 55,175 | 56,790
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 988,655 | 939,333 | 2,209,889 | 2,019,877
Total intersegment operating revenues | 19,366 | 29,325 | 42,251 | 91,443
Earnings (loss) on common stock | (35,333) | 61,453 | 4,544 | 94,715
Electric [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 63,492 | 61,949 | 174,410 | 169,780
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | 11,000 | 8,312 | 22,977 | 21,642
Natural gas distribution [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 80,069 | 92,440 | 504,805 | 627,450
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | (8,782) | (11,183) | 10,314 | 18,235
Pipeline and energy services [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 41,302 | 58,459 | 105,184 | 167,636
Total intersegment operating revenues | 7,046 | 10,591 | 36,393 | 47,836
Earnings (loss) on common stock | 3,273 | 5,221 | 21,884 | 16,913
Exploration and production [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 100,380 | 96,803 | 289,106 | 262,604
Total intersegment operating revenues | 8,076 | 23,956 | 25,114 | 74,889
Earnings (loss) on common stock | (87,748) | 22,497 | (56,860) | 60,093
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 641,500 | 619,134 | 1,229,731 | 1,138,280
Total intersegment operating revenues | 8,508 | 0 | 11,756 | 0
Earnings (loss) on common stock | 41,889 | 33,103 | 24,748 | 16,680
Construction services [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 246,358 | 222,822 | 688,368 | 617,699
Total intersegment operating revenues | 834 | 3,344 | 1,078 | 9,940
Earnings (loss) on common stock | 9,863 | 5,044 | 29,951 | 15,815
Other [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 417 | 574 | 2,684 | 1,294
Total intersegment operating revenues | 1,948 | 2,025 | 4,303 | 6,614
Earnings (loss) on common stock | 663 | 809 | 6,705 | 2,127
Intersegment Elimination [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 0 | 0 | 0 | 0
Total intersegment operating revenues | (26,412) | (39,916) | (78,644) | (139,279)
Earnings (loss) on common stock | $ 0 | $ 0 | $ 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 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 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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- 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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