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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,280,500 | $ 1,213,203 | $ 3,129,064 | $ 3,066,529
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations | 63,007 | 64,556 | 103,164 | 125,989
Income (loss) from discontinued operations, net of tax | (202,626) | 38,482 | (778,647) | 87,475
Earnings (loss) on common stock | (139,619) | 103,038 | (675,483) | 213,464
Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 185,224 | 184,793 | 806,986 | 863,569
Intersegment operating revenues | 3,740 | 3,728 | 31,365 | 27,938
Earnings (loss) on common stock before income (loss) from discontinued operations | 5,699 | 797 | 45,696 | 49,033
Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 1,095,276 | 1,028,410 | 2,322,078 | 2,202,960
Intersegment operating revenues | 4,880 | 25,517 | 26,151 | 52,489
Earnings (loss) on common stock before income (loss) from discontinued operations | 57,282 | 65,403 | 59,117 | 80,026
Electric [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 74,604 | 68,936 | 210,646 | 207,732
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations | 12,605 | 9,162 | 26,842 | 28,018
Natural gas distribution [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 89,520 | 96,185 | 553,058 | 616,496
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations | (12,298) | (12,252) | 3,777 | 10,516
Pipeline and energy services [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 21,100 | 19,672 | 43,282 | 39,341
Intersegment operating revenues | 3,740 | 3,728 | 31,365 | 27,938
Earnings (loss) on common stock before income (loss) from discontinued operations | 5,392 | 3,887 | 15,077 | 10,499
Pipeline and energy services [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 96,896 | 17,168 | 174,732 | 47,028
Intersegment operating revenues | 145 | 174 | 460 | 544
Earnings (loss) on common stock before income (loss) from discontinued operations | (14,903) | 1,173 | (22,175) | 4,699
Construction materials and contracting [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 774,288 | 740,496 | 1,475,585 | 1,339,371
Intersegment operating revenues | 244 | 6,322 | 2,450 | 18,445
Earnings (loss) on common stock before income (loss) from discontinued operations | 68,823 | 55,218 | 74,324 | 42,199
Construction services [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 223,676 | 270,313 | 670,594 | 815,313
Intersegment operating revenues | 2,112 | 16,420 | 17,298 | 27,431
Earnings (loss) on common stock before income (loss) from discontinued operations | 4,742 | 9,876 | 16,505 | 40,751
Other [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 416 | 433 | 1,167 | 1,248
Intersegment operating revenues | 2,379 | 2,601 | 5,943 | 6,069
Earnings (loss) on common stock before income (loss) from discontinued operations | (1,380) | (864) | (9,537) | (7,623)
Intersegment eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment operating revenues | (8,620) | (29,245) | (57,516) | (80,427)
Earnings (loss) on common stock before income (loss) from discontinued operations | $ 26 | $ (1,644) | $ (1,649) | $ (3,070)
Oil and natural gas properties [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percent of oil and natural gas production to be sold |  |  | 90.00% |

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