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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2017 | Mar. 31, 2016
Segment Reporting Information [Line Items] |  |
Revenues | $ 937,925 | $ 860,214
Intersegment operating revenues | 0 | 0
Earnings on common stock before income (loss) from discontinued operations | 35,467 | 31,694
Income (loss) from discontinued operations, net of tax | 1,687 | (18,036)
Loss from discontinued operations attributable to noncontrolling interest | 0 | (11,040)
Earnings on common stock | 37,154 | 24,698
Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 433,614 | 385,865
Intersegment operating revenues | 21,489 | 21,098
Earnings on common stock before income (loss) from discontinued operations | 46,751 | 41,648
Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 504,311 | 474,349
Intersegment operating revenues | 1,869 | 2,333
Earnings on common stock before income (loss) from discontinued operations | (13,457) | (9,954)
Electric [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 88,225 | 82,923
Intersegment operating revenues | 0 | 0
Earnings on common stock before income (loss) from discontinued operations | 14,333 | 11,119
Natural gas distribution [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 342,519 | 299,395
Intersegment operating revenues | 0 | 0
Earnings on common stock before income (loss) from discontinued operations | 27,861 | 25,241
Pipeline and midstream [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 2,870 | 3,547
Intersegment operating revenues | 21,489 | 21,098
Earnings on common stock before income (loss) from discontinued operations | 4,557 | 5,288
Pipeline and midstream [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 3,643 | 8,697
Intersegment operating revenues | 34 | 84
Earnings on common stock before income (loss) from discontinued operations | (628) | 1
Construction materials and contracting [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 200,776 | 209,852
Intersegment operating revenues | 86 | 118
Earnings on common stock before income (loss) from discontinued operations | (19,912) | (14,471)
Construction services [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 299,572 | 255,500
Intersegment operating revenues | 6 | 462
Earnings on common stock before income (loss) from discontinued operations | 7,362 | 5,974
Other [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 320 | 300
Intersegment operating revenues | 1,743 | 1,669
Earnings on common stock before income (loss) from discontinued operations | (279) | (1,458)
Intersegment eliminations [Member] |  |
Segment Reporting Information [Line Items] |  |
Intersegment operating revenues | (23,358) | (23,431)
Earnings on common stock before income (loss) from discontinued operations | $ 2,173 | $ 0

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