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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 1,280,787 | $ 1,272,548 | $ 3,321,678 | $ 3,278,112
Earnings on common stock before income (loss) from discontinued operations | 107,369 | 89,549 | 193,405 | 168,822
Income (loss) from discontinued operations, net of tax | (118) | (2,198) | 85 | (3,702)
Earnings on common stock | 107,251 | 87,351 | 193,490 | 165,120
Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 86,080 |  | 251,984 |
Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 92,248 |  | 554,452 |
Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 29,071 |  | 61,965 |
Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 743,763 |  | 1,466,435 |
Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 329,578 |  | 986,649 |
Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 47 |  | 193 |
Intersegment eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | (7,171) | (5,586) | (41,661) | (37,581)
Earnings on common stock before income (loss) from discontinued operations | 0 | 1,855 | 0 | 6,121
Intersegment eliminations | Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 |  | 0 |
Intersegment eliminations | Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 |  | 0 |
Intersegment eliminations | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | (3,187) |  | (31,485) |
Intersegment eliminations | Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | (165) |  | (501) |
Intersegment eliminations | Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | (782) |  | (1,332) |
Intersegment eliminations | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | (3,037) |  | (8,343) |
Total intersegment operating revenues |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Regulated operation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 200,617 | 206,936 | 851,761 | 866,035
Earnings on common stock before income (loss) from discontinued operations | 13,506 | 10,682 | 72,193 | 67,986
Regulated operation | Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 86,080 | 91,531 | 251,984 | 254,330
Earnings on common stock before income (loss) from discontinued operations | 15,284 | 15,712 | 37,500 | 37,904
Regulated operation | Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 92,248 | 92,253 | 554,452 | 566,364
Earnings on common stock before income (loss) from discontinued operations | (11,887) | (10,883) | 13,884 | 14,181
Regulated operation | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 22,289 | 23,152 | 45,325 | 45,341
Earnings on common stock before income (loss) from discontinued operations | 10,109 | 5,853 | 20,809 | 15,901
Regulated operation | Intersegment eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 3,070 | 3,081 | 31,252 | 30,923
Regulated operation | Intersegment eliminations | Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Regulated operation | Intersegment eliminations | Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0 | 0 | 0 | 0
Regulated operation | Intersegment eliminations | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 3,070 | 3,081 | 31,252 | 30,923
Nonregulated operation |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 1,080,170 | 1,065,612 | 2,469,917 | 2,412,077
Earnings on common stock before income (loss) from discontinued operations | 93,863 | 77,012 | 121,212 | 94,715
Nonregulated operation | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 6,782 | 5,356 | 16,640 | 13,518
Earnings on common stock before income (loss) from discontinued operations | 848 | 95 | 1,136 | (770)
Nonregulated operation | Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 743,763 | 686,010 | 1,466,435 | 1,388,212
Earnings on common stock before income (loss) from discontinued operations | 78,876 | 63,221 | 79,691 | 64,477
Nonregulated operation | Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 329,578 | 374,111 | 986,649 | 1,009,693
Earnings on common stock before income (loss) from discontinued operations | 9,278 | 13,144 | 38,457 | 32,896
Nonregulated operation | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 47 | 135 | 193 | 654
Earnings on common stock before income (loss) from discontinued operations | 4,861 | 552 | 1,928 | (1,888)
Nonregulated operation | Intersegment eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 4,101 | 2,505 | 10,409 | 6,658
Nonregulated operation | Intersegment eliminations | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 117 | 38 | 233 | 132
Nonregulated operation | Intersegment eliminations | Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 165 | 142 | 501 | 400
Nonregulated operation | Intersegment eliminations | Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 782 | 415 | 1,332 | 715
Nonregulated operation | Intersegment eliminations | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 3,037 | $ 1,910 | $ 8,343 | $ 5,411

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