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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | $ 1,272,548 | $ 1,208,567 | $ 3,278,112 | $ 3,112,730
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before loss from discontinued operations |  | 89,549 | 88,215 | 168,822 | 166,036
Loss from discontinued operations, net of tax | [1] | (2,198) | (5,400) | (3,702) | (299,538)
Loss from discontinued operations attributable to noncontrolling interest |  | 0 | 0 | 0 | (131,691)
Earnings (loss) on common stock |  | 87,351 | 82,815 | 165,120 | (1,811)
Intersegment eliminations [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Intersegment operating revenues |  | (5,586) | (5,681) | (37,581) | (37,583)
Earnings on common stock before loss from discontinued operations | [1] | 1,855 | 5,599 | 6,121 | 5,599
Regulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 206,936 | 192,079 | 866,035 | 783,997
Intersegment operating revenues |  | 3,081 | 3,278 | 30,923 | 30,969
Earnings on common stock before loss from discontinued operations |  | 10,682 | 5,564 | 67,986 | 53,021
Regulated operation [Member] | Electric [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 91,531 | 82,156 | 254,330 | 238,911
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before loss from discontinued operations |  | 15,712 | 12,699 | 37,904 | 31,840
Regulated operation [Member] | Natural gas distribution [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 92,253 | 87,941 | 566,364 | 500,106
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before loss from discontinued operations |  | (10,883) | (12,524) | 14,181 | 4,940
Regulated operation [Member] | Pipeline and midstream [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 23,152 | 21,982 | 45,341 | 44,980
Intersegment operating revenues |  | 3,081 | 3,278 | 30,923 | 30,969
Earnings on common stock before loss from discontinued operations |  | 5,853 | 5,389 | 15,901 | 16,241
Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 1,065,612 | 1,016,488 | 2,412,077 | 2,328,733
Intersegment operating revenues |  | 2,505 | 2,403 | 6,658 | 6,614
Earnings on common stock before loss from discontinued operations |  | 77,012 | 77,052 | 94,715 | 107,416
Nonregulated operation [Member] | Pipeline and midstream [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 5,356 | 10,732 | 13,518 | 29,697
Intersegment operating revenues |  | 38 | 41 | 132 | 161
Earnings on common stock before loss from discontinued operations |  | 95 | 1,304 | (770) | 2,043
Nonregulated operation [Member] | Construction materials and contracting [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 686,010 | 724,535 | 1,388,212 | 1,475,643
Intersegment operating revenues |  | 142 | 155 | 400 | 370
Earnings on common stock before loss from discontinued operations |  | 63,221 | 69,523 | 64,477 | 88,747
Nonregulated operation [Member] | Construction services [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 374,111 | 280,801 | 1,009,693 | 822,226
Intersegment operating revenues |  | 415 | 3 | 715 | 541
Earnings on common stock before loss from discontinued operations |  | 13,144 | 7,234 | 32,896 | 20,198
Nonregulated operation [Member] | Other [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 135 | 420 | 654 | 1,167
Intersegment operating revenues |  | 1,910 | 2,204 | 5,411 | 5,542
Earnings on common stock before loss from discontinued operations |  | $ 552 | $ (1,009) | $ (1,888) | $ (3,572)
[1] | Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

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