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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,208,567 | $ 1,198,342 | $ 3,112,730 | $ 2,997,225
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | 88,215 | 73,715 | 166,036 | 119,974
Loss from discontinued operations, net of tax | (5,400) | (223,112) | (299,538) | (816,517)
Loss from discontinued operations attributable to noncontrolling interest | 0 | (9,778) | (131,691) | (21,060)
Earnings (loss) on common stock | 82,815 | (139,619) | (1,811) | (675,483)
Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 192,079 | 185,417 | 783,997 | 807,585
Intersegment operating revenues | 3,278 | 3,740 | 30,969 | 31,365
Earnings (loss) on common stock before loss from discontinued operations | 5,564 | 5,699 | 53,021 | 45,696
Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 1,016,488 | 1,012,925 | 2,328,733 | 2,189,640
Intersegment operating revenues | 2,403 | 4,880 | 6,614 | 26,151
Earnings (loss) on common stock before loss from discontinued operations | 77,052 | 62,775 | 107,416 | 70,771
Electric [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 82,156 | 74,604 | 238,911 | 210,646
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | 12,699 | 12,605 | 31,840 | 26,842
Natural gas distribution [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 87,941 | 89,520 | 500,106 | 553,058
Intersegment operating revenues | 0 | 0 | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | (12,524) | (12,298) | 4,940 | 3,777
Pipeline and midstream [Member] | Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 21,982 | 21,293 | 44,980 | 43,881
Intersegment operating revenues | 3,278 | 3,740 | 30,969 | 31,365
Earnings (loss) on common stock before loss from discontinued operations | 5,389 | 5,392 | 16,241 | 15,077
Pipeline and midstream [Member] | Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 10,732 | 14,545 | 29,697 | 42,294
Intersegment operating revenues | 41 | 145 | 161 | 460
Earnings (loss) on common stock before loss from discontinued operations | 1,304 | (8,587) | 2,043 | (8,498)
Construction materials and contracting [Member] | Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 724,535 | 774,288 | 1,475,643 | 1,475,585
Intersegment operating revenues | 155 | 244 | 370 | 2,450
Earnings (loss) on common stock before loss from discontinued operations | 69,523 | 68,823 | 88,747 | 74,324
Construction services [Member] | Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 280,801 | 223,676 | 822,226 | 670,594
Intersegment operating revenues | 3 | 2,112 | 541 | 17,298
Earnings (loss) on common stock before loss from discontinued operations | 7,234 | 4,742 | 20,198 | 16,505
Other [Member] | Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 420 | 416 | 1,167 | 1,167
Intersegment operating revenues | 2,204 | 2,379 | 5,542 | 5,943
Earnings (loss) on common stock before loss from discontinued operations | (1,009) | (2,203) | (3,572) | (11,560)
Intersegment eliminations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment operating revenues | (5,681) | (8,620) | (37,583) | (57,516)
Earnings (loss) on common stock before loss from discontinued operations | $ 5,599 | $ 5,241 | $ 5,599 | $ 3,507

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