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Business segment data (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2012 | Jun. 30, 2011 | Jun. 30, 2012 | Jun. 30, 2011
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | $ 967,962 | $ 930,757 | $ 1,820,769 | $ 1,832,562
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | 53,942 | 44,896 | 89,561 | 87,702
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 204,455 | 274,538 | 599,533 | 752,018
Total intersegment operating revenues | 8,937 | 12,504 | 29,347 | 37,245
Earnings (loss) on common stock | 13,859 | 11,481 | 49,686 | 54,440
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 763,507 | 656,219 | 1,221,236 | 1,080,544
Total intersegment operating revenues | 11,055 | 33,072 | 22,885 | 62,118
Earnings (loss) on common stock | 40,083 | 33,415 | 39,875 | 33,262
Electric [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 52,955 | 49,986 | 110,918 | 107,831
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | 4,419 | 4,807 | 11,978 | 13,331
Natural gas distribution [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 116,844 | 164,626 | 424,733 | 535,010
Total intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | (6,411) | 1,902 | 19,097 | 29,418
Pipeline and energy services [Member] | Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 34,656 | 59,926 | 63,882 | 109,177
Total intersegment operating revenues | 8,937 | 12,504 | 29,347 | 37,245
Earnings (loss) on common stock | 15,851 | 4,772 | 18,611 | 11,691
Exploration and production [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 100,232 | 87,390 | 188,727 | 165,801
Total intersegment operating revenues | 5,711 | 25,392 | 17,038 | 50,933
Earnings (loss) on common stock | 17,957 | 21,326 | 30,887 | 37,596
Construction materials and contracting [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 438,963 | 375,613 | 588,232 | 519,146
Total intersegment operating revenues | 3,097 | 0 | 3,248 | 0
Earnings (loss) on common stock | 7,791 | 4,980 | (17,141) | (16,423)
Construction services [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 223,858 | 192,697 | 442,010 | 394,877
Total intersegment operating revenues | 219 | 5,379 | 244 | 6,596
Earnings (loss) on common stock | 8,684 | 6,138 | 20,087 | 10,771
Other [Member] | Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 454 | 519 | 2,267 | 720
Total intersegment operating revenues | 2,028 | 2,301 | 2,355 | 4,589
Earnings (loss) on common stock | 5,651 | 971 | 6,042 | 1,318
Intersegment Elimination [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total external operating revenues | 0 | 0 | 0 | 0
Total intersegment operating revenues | (19,992) | (45,576) | (52,232) | (99,363)
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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