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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 986,215 | $ 952,564 | $ 1,848,564 | $ 1,853,326
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock | (229,775) | 53,935 | (535,863) | 110,426
Earnings (loss) on common stock before income (loss) from discontinued operations | 21,640 | 30,054 | 40,157 | 61,433
Income (loss) from discontinued operations, net of tax | (251,415) | 23,881 | (576,020) | 48,993
Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 215,472 | 226,472 | 621,762 | 678,775
Intersegment operating revenues | 6,564 | 6,937 | 27,625 | 24,210
Earnings (loss) on common stock before income (loss) from discontinued operations | 4,864 | 6,943 | 39,997 | 48,236
Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 770,743 | 726,092 | 1,226,802 | 1,174,551
Intersegment operating revenues | 6,650 | 17,300 | 21,270 | 26,972
Earnings (loss) on common stock before income (loss) from discontinued operations | 17,460 | 24,065 | 1,835 | 14,624
Electric [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 64,265 | 65,149 | 136,041 | 138,796
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations | 5,910 | 7,823 | 14,237 | 18,856
Natural gas distribution [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 132,965 | 146,077 | 463,538 | 520,311
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations | (5,375) | (4,494) | 16,075 | 22,768
Pipeline and energy services [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 18,242 | 15,246 | 22,183 | 19,668
Intersegment operating revenues | 6,564 | 6,937 | 27,625 | 24,210
Earnings (loss) on common stock before income (loss) from discontinued operations | 4,329 | 3,614 | 9,685 | 6,612
Pipeline and energy services [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 63,131 | 16,044 | 77,834 | 29,859
Intersegment operating revenues | 110 | 177 | 316 | 371
Earnings (loss) on common stock before income (loss) from discontinued operations | (5,933) | 2,175 | (7,271) | 3,526
Construction materials and contracting [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 495,640 | 434,452 | 701,298 | 598,875
Intersegment operating revenues | 1,257 | 8,106 | 2,205 | 12,123
Earnings (loss) on common stock before income (loss) from discontinued operations | 20,136 | 10,554 | 5,501 | (13,019)
Construction services [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 211,515 | 275,109 | 446,918 | 545,002
Intersegment operating revenues | 3,491 | 7,273 | 15,186 | 11,010
Earnings (loss) on common stock before income (loss) from discontinued operations | 7,003 | 14,307 | 11,763 | 30,875
Other [Member] | Unregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 457 | 487 | 752 | 815
Intersegment operating revenues | 1,792 | 1,744 | 3,563 | 3,468
Earnings (loss) on common stock before income (loss) from discontinued operations | (3,746) | (2,971) | (8,158) | (6,758)
Intersegment eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment operating revenues | (13,214) | (24,237) | (48,895) | (51,182)
Earnings (loss) on common stock before income (loss) from discontinued operations | $ (684) | $ (954) | $ (1,675) | $ (1,427)

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