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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2016 | Mar. 31, 2015
Segment Reporting Information [Line Items] |  |
Revenues | $ 905,153 | $ 862,349
Intersegment operating revenues | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | 25,533 | 18,517
Loss from discontinued operations, net of tax | (835) | (324,605)
Earnings (loss) on common stock | 24,698 | (306,088)
Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 385,738 | 406,289
Intersegment operating revenues | 21,225 | 21,261
Earnings (loss) on common stock before loss from discontinued operations | 41,648 | 35,135
Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 519,415 | 456,060
Intersegment operating revenues | 2,333 | 14,740
Earnings (loss) on common stock before loss from discontinued operations | (16,233) | (15,627)
Electric [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 82,923 | 71,776
Intersegment operating revenues | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | 11,119 | 8,328
Natural gas distribution [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 299,395 | 330,573
Intersegment operating revenues | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | 25,241 | 21,450
Pipeline and midstream [Member] | Regulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 3,420 | 3,940
Intersegment operating revenues | 21,225 | 21,261
Earnings (loss) on common stock before loss from discontinued operations | 5,288 | 5,357
Pipeline and midstream [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 8,697 | 13,000
Intersegment operating revenues | 84 | 325
Earnings (loss) on common stock before loss from discontinued operations | 1 | 1,055
Construction materials and contracting [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 209,852 | 205,658
Intersegment operating revenues | 118 | 948
Earnings (loss) on common stock before loss from discontinued operations | (14,471) | (14,635)
Construction services [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 255,500 | 235,403
Intersegment operating revenues | 462 | 11,695
Earnings (loss) on common stock before loss from discontinued operations | 5,974 | 4,760
Refining [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 45,066 | 1,704
Intersegment operating revenues | 0 | 0
Earnings (loss) on common stock before loss from discontinued operations | (7,187) | (2,394)
Other [Member] | Nonregulated operation [Member] |  |
Segment Reporting Information [Line Items] |  |
Revenues | 300 | 295
Intersegment operating revenues | 1,669 | 1,772
Earnings (loss) on common stock before loss from discontinued operations | (550) | (4,413)
Intersegment eliminations [Member] |  |
Segment Reporting Information [Line Items] |  |
Intersegment operating revenues | (23,558) | (36,001)
Earnings (loss) on common stock before loss from discontinued operations | $ 118 | $ (991)

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