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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | $ 976,293 | $ 937,925 |
Intersegment operating revenues |  | 0 | 0 |
Earnings on common stock before income from discontinued operations |  | 41,960 | 35,467 |
Income from discontinued operations, net of tax |  | 477 | 1,687 | [1]
Earnings on common stock |  | 42,437 | 37,154 |
Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 87,404 |  |
Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 332,664 |  |
Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 8,833 |  |
Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 213,284 |  |
Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 334,050 |  |
Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 58 |  |
Intersegment eliminations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Intersegment operating revenues |  | (24,509) | (23,358) |
Earnings on common stock before income from discontinued operations | [1] | 0 | 2,173 |
Regulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 424,459 | 433,614 |
Intersegment operating revenues |  | 21,735 | 21,489 |
Earnings on common stock before income from discontinued operations |  | 51,166 | 46,751 |
Regulated operation [Member] | Electric |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 87,404 | 88,225 |
Intersegment operating revenues |  | 0 | 0 |
Earnings on common stock before income from discontinued operations |  | 13,084 | 14,333 |
Regulated operation [Member] | Natural gas distribution |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 332,664 | 342,519 |
Intersegment operating revenues |  | 0 | 0 |
Earnings on common stock before income from discontinued operations |  | 32,623 | 27,861 |
Regulated operation [Member] | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 4,391 | 2,870 |
Intersegment operating revenues |  | 21,735 | 21,489 |
Earnings on common stock before income from discontinued operations |  | 5,459 | 4,557 |
Nonregulated operation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 551,834 | 504,311 |
Intersegment operating revenues |  | 2,774 | 1,869 |
Earnings on common stock before income from discontinued operations |  | (9,206) | (13,457) |
Nonregulated operation [Member] | Pipeline and midstream |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 4,442 | 3,643 |
Intersegment operating revenues |  | 24 | 34 |
Earnings on common stock before income from discontinued operations |  | (179) | (628) |
Nonregulated operation [Member] | Construction materials and contracting |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 213,284 | 200,776 |
Intersegment operating revenues |  | 101 | 86 |
Earnings on common stock before income from discontinued operations |  | (23,521) | (19,912) |
Nonregulated operation [Member] | Construction services |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 334,050 | 299,572 |
Intersegment operating revenues |  | 11 | 6 |
Earnings on common stock before income from discontinued operations |  | 15,090 | 7,362 |
Nonregulated operation [Member] | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues |  | 58 | 320 |
Intersegment operating revenues |  | 2,638 | 1,743 |
Earnings on common stock before income from discontinued operations |  | $ (596) | $ (279) |
[1] | Includes an elimination for the presentation of income tax adjustments between continuing and discontinued operations.

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