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Business segment data (Details) (USD $) In Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2011 | Jun. 30, 2010 | Jun. 30, 2011 | Jun. 30, 2010
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | $ 930,757 | $ 906,444 | $ 1,832,562 | $ 1,741,221
Earnings on common stock | 44,896 | 48,767 | 87,702 | 90,367
Income (loss) on discontinued operations | (168) | 0 | 280 | 0
Regulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 274,538 | 272,177 | 752,018 | 732,422
Inter-segment operating revenues | 12,504 | 14,143 | 37,245 | 41,228
Earnings on common stock | 11,481 | 14,562 | 54,440 | 52,580
Regulated Operation [Member] | Electric [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 49,986 | 45,683 | 107,831 | 95,379
Inter-segment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 4,807 | 4,947 | 13,331 | 10,832
Regulated Operation [Member] | Natural gas distribution [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 164,626 | 160,138 | 535,010 | 509,162
Inter-segment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 1,902 | 74 | 29,418 | 23,416
Regulated Operation [Member] | Pipeline and energy services [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 59,926 | 66,356 | 109,177 | 127,881
Inter-segment operating revenues | 12,504 | 14,143 | 37,245 | 41,228
Earnings on common stock | 4,772 | 9,541 | 11,691 | 18,332
Unregulated Operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 656,219 | 634,267 | 1,080,544 | 1,008,799
Inter-segment operating revenues | 33,072 | 28,621 | 62,118 | 66,810
Earnings on common stock | 33,415 | 34,205 | 33,262 | 37,787
Unregulated Operation [Member] | Construction services [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 192,697 | 188,182 | 394,877 | 341,247
Inter-segment operating revenues | 5,379 | 8 | 6,596 | 32
Earnings on common stock | 6,138 | 2,923 | 10,771 | 3,051
Unregulated Operation [Member] | Natural gas and oil production [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 87,390 | 84,406 | 165,801 | 156,066
Inter-segment operating revenues | 25,392 | 26,400 | 50,933 | 62,327
Earnings on common stock | 21,326 | 24,035 | 37,596 | 46,246
Unregulated Operation [Member] | Construction materials and contracting [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 375,613 | 361,625 | 519,146 | 511,432
Inter-segment operating revenues | 0 | 0 | 0 | 0
Earnings on common stock | 4,980 | 5,659 | (16,423) | (14,478)
Unregulated Operation [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 519 | 54 | 720 | 54
Inter-segment operating revenues | 2,301 | 2,213 | 4,589 | 4,451
Earnings on common stock | 971 | 1,588 | 1,318 | 2,968
Intersegment Elimination [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
External operating revenues | 0 | 0 | 0 | 0
Inter-segment operating revenues | (45,576) | (42,764) | (99,363) | (108,038)
Earnings 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 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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