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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | $ 1,067,639 | $ 1,043,948 | $ 2,005,565 | $ 1,904,163
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before income (loss) from discontinued operations |  | 43,805 | 46,127 | 79,272 | 77,821
Loss from discontinued operations, net of tax | [1] | (3,190) | (276,102) | (1,504) | (294,138)
Loss from discontinued operations attributable to noncontrolling interest |  | 0 | (120,651) | 0 | (131,691)
Earnings (loss) on common stock |  | 40,615 | (109,324) | 77,768 | (84,626)
Regulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 225,485 | 206,052 | 659,100 | 591,918
Intersegment operating revenues |  | 6,353 | 6,594 | 27,841 | 27,691
Earnings on common stock before income (loss) from discontinued operations |  | 10,554 | 5,809 | 57,303 | 47,457
Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 842,154 | 837,896 | 1,346,465 | 1,312,245
Intersegment operating revenues |  | 2,284 | 1,879 | 4,153 | 4,211
Earnings on common stock before income (loss) from discontinued operations |  | 31,158 | 40,318 | 17,703 | 30,364
Electric [Member] | Regulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 74,574 | 73,832 | 162,799 | 156,755
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before income (loss) from discontinued operations |  | 7,859 | 8,022 | 22,191 | 19,141
Natural gas distribution [Member] | Regulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 131,592 | 112,770 | 474,111 | 412,165
Intersegment operating revenues |  | 0 | 0 | 0 | 0
Earnings on common stock before income (loss) from discontinued operations |  | (2,797) | (7,777) | 25,064 | 17,464
Pipeline and midstream [Member] | Regulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 19,319 | 19,450 | 22,190 | 22,998
Intersegment operating revenues |  | 6,353 | 6,594 | 27,841 | 27,691
Earnings on common stock before income (loss) from discontinued operations |  | 5,492 | 5,564 | 10,048 | 10,852
Pipeline and midstream [Member] | Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 4,520 | 10,268 | 8,163 | 18,966
Intersegment operating revenues |  | 59 | 36 | 93 | 119
Earnings on common stock before income (loss) from discontinued operations |  | (238) | 737 | (865) | 739
Construction materials and contracting [Member] | Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 501,426 | 541,257 | 702,203 | 751,108
Intersegment operating revenues |  | 172 | 97 | 258 | 215
Earnings on common stock before income (loss) from discontinued operations |  | 21,168 | 33,696 | 1,255 | 19,225
Construction services [Member] | Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 336,009 | 285,924 | 635,580 | 541,424
Intersegment operating revenues |  | 295 | 77 | 301 | 539
Earnings on common stock before income (loss) from discontinued operations |  | 12,391 | 6,990 | 19,753 | 12,964
Other [Member] | Nonregulated operation [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Revenues |  | 199 | 447 | 519 | 747
Intersegment operating revenues |  | 1,758 | 1,669 | 3,501 | 3,338
Earnings on common stock before income (loss) from discontinued operations |  | (2,163) | (1,105) | (2,440) | (2,564)
Intersegment Eliminations [Member] |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |
Intersegment operating revenues |  | (8,637) | (8,473) | (31,994) | (31,902)
Earnings on common stock before income (loss) from discontinued operations | [1] | $ 2,093 | $ 0 | $ 4,266 | $ 0
[1] | Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.

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