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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  | $ 1,165,239 | $ 1,272,548 | $ 1,067,639 | $ 937,925 | $ 1,016,099 | $ 1,208,567 | $ 1,043,948 | $ 860,214 | $ 4,443,351 | $ 4,128,828 | $ 4,014,052
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 207,486 | 216,318 | 211,747
Interest expense |  |  |  |  |  |  |  |  |  | 82,788 | 87,848 | 91,179
Income taxes |  |  |  |  |  |  |  |  |  | 65,041 | 93,132 | 70,664
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 284,215 | 232,411 | 175,704
Loss from discontinued operations, net of tax | [1] |  |  |  |  |  |  |  |  | (3,783) | (300,354) | (834,080)
Loss from discontinued operations attributable to noncontrolling interest |  |  |  |  |  |  |  |  |  | 0 | (131,691) | (35,256)
Total earnings (loss) on common stock |  |  |  |  |  |  |  |  |  | 280,432 | 63,748 | (623,120)
Capital expenditures | [2] |  |  |  |  |  |  |  |  | 351,924 | 372,420 | 572,134
Total assets |  | 6,334,666 |  |  |  | 6,284,467 |  |  |  | 6,334,666 | 6,284,467 | 6,565,154
Property, plant and equipment |  | 6,770,829 |  |  |  | 6,510,229 |  |  |  | 6,770,829 | 6,510,229 |
Less accumulated depreciation, depletion and amortization |  | 2,691,641 |  |  |  | 2,578,902 |  |  |  | 2,691,641 | 2,578,902 | 2,489,322
Net property, plant and equipment |  | 4,079,188 |  |  |  | 3,931,327 |  |  |  | 4,079,188 | 3,931,327 | 3,898,380
Additional information [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Net noncash capital expenditure-related accounts payable and AFUDC |  |  |  |  |  |  |  |  |  | 10,500 | (15,800) | 35,300
Electric: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 47,715 | 50,220 | 37,583
Interest expense |  |  |  |  |  |  |  |  |  | 25,377 | 24,982 | 17,421
Income taxes |  |  |  |  |  |  |  |  |  | 7,699 | 1,449 | 11,523
Capital expenditures |  |  |  |  |  |  |  |  |  | 109,107 | 111,134 | 332,876
Total assets | [3] | 1,470,922 |  |  |  | 1,406,694 |  |  |  | 1,470,922 | 1,406,694 | 1,325,858
Property, plant and equipment | [3] | 1,982,264 |  |  |  | 1,888,613 |  |  |  | 1,982,264 | 1,888,613 | 1,786,148
Natural gas distribution: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 69,381 | 65,426 | 64,756
Interest expense |  |  |  |  |  |  |  |  |  | 31,234 | 30,405 | 29,471
Income taxes |  |  |  |  |  |  |  |  |  | 22,756 | 9,181 | 11,377
Capital expenditures |  |  |  |  |  |  |  |  |  | 146,981 | 126,272 | 130,793
Total assets | [3] | 2,201,081 |  |  |  | 2,099,296 |  |  |  | 2,201,081 | 2,099,296 | 2,038,433
Property, plant and equipment | [3] | 2,319,845 |  |  |  | 2,179,413 |  |  |  | 2,319,845 | 2,179,413 | 2,076,581
Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 16,788 | 24,885 | 27,981
Interest expense |  |  |  |  |  |  |  |  |  | 4,990 | 7,903 | 9,895
Income taxes |  |  |  |  |  |  |  |  |  | 12,281 | 12,408 | 7,505
Capital expenditures |  |  |  |  |  |  |  |  |  | 31,054 | 34,467 | 18,315
Total assets |  | 566,295 |  |  |  | 550,615 |  |  |  | 566,295 | 550,615 | 591,651
Property, plant and equipment |  | 700,284 |  |  |  | 672,199 |  |  |  | 700,284 | 672,199 | 758,729
Construction materials and contracting: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 55,862 | 58,413 | 65,937
Interest expense |  |  |  |  |  |  |  |  |  | 14,778 | 15,265 | 15,183
Income taxes |  |  |  |  |  |  |  |  |  | 5,405 | 60,625 | 41,619
Capital expenditures |  |  |  |  |  |  |  |  |  | 44,302 | 37,845 | 48,126
Total assets |  | 1,238,696 |  |  |  | 1,220,459 |  |  |  | 1,238,696 | 1,220,459 | 1,261,963
Property, plant and equipment |  | 1,560,048 |  |  |  | 1,549,375 |  |  |  | 1,560,048 | 1,549,375 | 1,553,428
Construction services: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 15,739 | 15,307 | 13,420
Interest expense |  |  |  |  |  |  |  |  |  | 3,742 | 4,059 | 3,959
Income taxes |  |  |  |  |  |  |  |  |  | 25,558 | 17,748 | 16,432
Capital expenditures |  |  |  |  |  |  |  |  |  | 18,630 | 60,344 | 38,269
Total assets |  | 591,382 |  |  |  | 513,093 |  |  |  | 591,382 | 513,093 | 442,845
Property, plant and equipment |  | 177,265 |  |  |  | 171,361 |  |  |  | 177,265 | 171,361 | 163,279
Other |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 2,001 | 2,067 | 2,070
Interest expense |  |  |  |  |  |  |  |  |  | 3,564 | 5,854 | 15,853
Income taxes |  |  |  |  |  |  |  |  |  | (1,809) | (2,028) | (9,834)
Capital expenditures |  |  |  |  |  |  |  |  |  | 1,850 | 2,358 | 3,755
Total assets | [4] | 261,419 |  |  |  | 283,255 |  |  |  | 261,419 | 283,255 | 287,940
Property, plant and equipment |  | 31,123 |  |  |  | 49,268 |  |  |  | 31,123 | 49,268 | 49,537
Discontinued operations, held-for-sale |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total assets |  | $ 4,871 |  |  |  | $ 211,055 |  |  |  | 4,871 | 211,055 | 616,464
Eliminations |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | (58,060) | (57,430) | (78,786)
Interest expense |  |  |  |  |  |  |  |  |  | (897) | (620) | (603)
Income taxes |  |  |  |  |  |  |  |  |  | (6,849) | (6,251) | (7,958)
Earnings (loss) on common stock: | [1] |  |  |  |  |  |  |  |  | 6,849 | 6,251 | 5,016
Regulated: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 1,244,759 | 1,141,454 | 1,149,038
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 48,867 | 48,794 | 49,065
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 102,211 | 91,384 | 80,201
Regulated: | Electric: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 342,805 | 322,356 | 280,615
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 49,366 | 42,222 | 35,914
Regulated: | Natural gas distribution: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 848,388 | 766,115 | 817,419
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 32,225 | 27,102 | 23,607
Regulated: | Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 53,566 | 52,983 | 51,004
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 48,867 | 48,794 | 49,065
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 20,620 | 22,060 | 20,680
Nonregulated: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 3,198,592 | 2,987,374 | 2,865,014
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 9,193 | 8,636 | 29,721
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 175,155 | 134,776 | 90,487
Nonregulated: | Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 19,602 | 39,602 | 54,281
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 178 | 223 | 554
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | (127) | 1,375 | (7,430)
Nonregulated: | Construction materials and contracting: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 1,811,964 | 1,873,696 | 1,901,530
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 565 | 574 | 2,752
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 123,398 | 102,687 | 89,096
Nonregulated: | Construction services: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 1,366,317 | 1,072,663 | 907,767
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 1,285 | 609 | 18,660
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | 53,306 | 33,945 | 23,762
Nonregulated: | Other |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  |  | 709 | 1,413 | 1,436
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 7,165 | 7,230 | 7,755
Earnings (loss) on common stock: |  |  |  |  |  |  |  |  |  | $ (1,422) | $ (3,231) | $ (14,941)
[1] | Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.
[2] | Capital expenditures for 2017, 2016 and 2015 include noncash capital expenditure-related accounts payable and AFUDC, totaling $10.5 million, $(15.8) million and $35.3 million, respectively.
[3] | Includes allocations of common utility property
[4] | Includes assets not directly assignable to a business (i.e. cash and cash equivalents, certain accounts receivable, certain investments and other miscellaneous current and deferred assets)

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