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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  | $ 1,016,099 | $ 1,208,567 | $ 1,043,948 | $ 860,214 | $ 1,016,826 | $ 1,198,342 | $ 938,039 | $ 860,845 | $ 4,128,828 | $ 4,014,052 | $ 4,115,073
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 216,318 | 211,747 | 203,084
Interest expense |  |  |  |  |  |  |  |  |  | 87,848 | 91,179 | 86,871
Income taxes |  |  |  |  |  |  |  |  |  | 93,132 | 70,664 | 64,422
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 232,411 | 175,704 | 184,342
Income (loss) from discontinued operations, net of tax | [1] |  |  |  |  |  |  |  |  | (300,354) | (834,080) | 109,311
Loss from discontinued operations attributable to noncontrolling interest |  |  |  |  |  |  |  |  |  | (131,691) | (35,256) | (3,895)
Total earnings (loss) on common stock |  |  |  |  |  |  |  |  |  | 63,748 | (623,120) | 297,548
Capital expenditures | [2] |  |  |  |  |  |  |  |  | 372,420 | 572,134 | 434,457
Total assets |  | 6,284,467 |  |  |  | 6,565,154 |  |  |  | 6,284,467 | 6,565,154 | 7,805,405
Property, plant and equipment |  | 6,510,229 |  |  |  | 6,387,702 |  |  |  | 6,510,229 | 6,387,702 |
Less accumulated depreciation, depletion and amortization |  | 2,578,902 |  |  |  | 2,489,322 |  |  |  | 2,578,902 | 2,489,322 | 2,385,202
Net property, plant and equipment |  | 3,931,327 |  |  |  | 3,898,380 |  |  |  | 3,931,327 | 3,898,380 | 3,520,320
Additional information [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Net noncash capital expenditure-related accounts payable and AFUDC |  |  |  |  |  |  |  |  |  | (15,800) | 35,300 | 5,100
Electric: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 50,220 | 37,583 | 35,008
Interest expense |  |  |  |  |  |  |  |  |  | 24,982 | 17,421 | 15,595
Income taxes |  |  |  |  |  |  |  |  |  | 1,449 | 11,523 | 12,442
Capital expenditures |  |  |  |  |  |  |  |  |  | 111,134 | 332,876 | 185,121
Total assets | [3] | 1,406,694 |  |  |  | 1,325,858 |  |  |  | 1,406,694 | 1,325,858 | 1,028,001
Property, plant and equipment | [3] | 1,888,613 |  |  |  | 1,786,148 |  |  |  | 1,888,613 | 1,786,148 | 1,457,101
Natural gas distribution: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 65,426 | 64,756 | 54,700
Interest expense |  |  |  |  |  |  |  |  |  | 30,405 | 29,471 | 27,217
Income taxes |  |  |  |  |  |  |  |  |  | 9,181 | 11,377 | 11,350
Capital expenditures |  |  |  |  |  |  |  |  |  | 126,272 | 130,793 | 120,613
Total assets | [3] | 2,099,296 |  |  |  | 2,038,433 |  |  |  | 2,099,296 | 2,038,433 | 1,935,271
Property, plant and equipment | [3] | 2,179,413 |  |  |  | 2,076,581 |  |  |  | 2,179,413 | 2,076,581 | 1,904,759
Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 24,885 | 27,981 | 29,749
Interest expense |  |  |  |  |  |  |  |  |  | 7,903 | 9,895 | 9,946
Income taxes |  |  |  |  |  |  |  |  |  | 12,408 | 7,505 | 12,232
Capital expenditures |  |  |  |  |  |  |  |  |  | 34,467 | 18,315 | 61,754
Total assets |  | 550,615 |  |  |  | 591,651 |  |  |  | 550,615 | 591,651 | 651,925
Property, plant and equipment |  | 672,199 |  |  |  | 758,729 |  |  |  | 672,199 | 758,729 | 818,388
Construction materials and contracting: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 58,413 | 65,937 | 68,557
Interest expense |  |  |  |  |  |  |  |  |  | 15,265 | 15,183 | 16,368
Income taxes |  |  |  |  |  |  |  |  |  | 60,625 | 41,619 | 18,586
Capital expenditures |  |  |  |  |  |  |  |  |  | 37,845 | 48,126 | 37,896
Total assets |  | 1,220,459 |  |  |  | 1,261,963 |  |  |  | 1,220,459 | 1,261,963 | 1,260,534
Property, plant and equipment |  | 1,549,375 |  |  |  | 1,553,428 |  |  |  | 1,549,375 | 1,553,428 | 1,529,942
Construction services: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 15,307 | 13,420 | 12,874
Interest expense |  |  |  |  |  |  |  |  |  | 4,059 | 3,959 | 4,176
Income taxes |  |  |  |  |  |  |  |  |  | 17,748 | 16,432 | 24,753
Capital expenditures |  |  |  |  |  |  |  |  |  | 60,344 | 38,269 | 26,942
Total assets |  | 513,093 |  |  |  | 442,845 |  |  |  | 513,093 | 442,845 | 437,322
Property, plant and equipment |  | 171,361 |  |  |  | 163,279 |  |  |  | 171,361 | 163,279 | 144,395
Other |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  |  | 2,067 | 2,070 | 2,196
Interest expense |  |  |  |  |  |  |  |  |  | 5,854 | 15,853 | 13,823
Income taxes |  |  |  |  |  |  |  |  |  | (2,028) | (9,834) | (11,136)
Capital expenditures |  |  |  |  |  |  |  |  |  | 2,358 | 3,755 | 2,131
Total assets | [4] | 283,255 |  |  |  | 287,940 |  |  |  | 283,255 | 287,940 | 315,495
Property, plant and equipment |  | 49,268 |  |  |  | 49,537 |  |  |  | 49,268 | 49,537 | 50,937
Discontinued operations, held-for-sale |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total assets |  | $ 211,055 |  |  |  | $ 616,464 |  |  |  | 211,055 | 616,464 | 2,176,857
Eliminations |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | (57,430) | (78,786) | (136,302)
Interest expense |  |  |  |  |  |  |  |  |  | (620) | (603) | (254)
Income taxes |  |  |  |  |  |  |  |  |  | (6,251) | (7,958) | (3,805)
Earnings (loss) on common stock before income (loss) from discontinued operations | [1] |  |  |  |  |  |  |  |  | 6,251 | 5,016 | (6,095)
Regulated: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 1,141,454 | 1,149,038 | 1,246,903
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 48,794 | 49,065 | 45,013
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 91,384 | 80,201 | 82,655
Regulated: | Electric: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 322,356 | 280,615 | 277,874
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 42,222 | 35,914 | 36,731
Regulated: | Natural gas distribution: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 766,115 | 817,419 | 921,986
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 27,102 | 23,607 | 30,484
Regulated: | Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 52,983 | 51,004 | 47,043
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 48,794 | 49,065 | 45,013
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 22,060 | 20,680 | 15,440
Nonregulated: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 2,987,374 | 2,865,014 | 2,868,170
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 8,636 | 29,721 | 91,289
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 134,776 | 90,487 | 107,782
Nonregulated: | Pipeline and midstream: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 39,602 | 54,281 | 64,494
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 223 | 554 | 742
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 1,375 | (7,430) | 9,226
Nonregulated: | Construction materials and contracting: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 1,873,696 | 1,901,530 | 1,740,089
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 574 | 2,752 | 25,241
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 102,687 | 89,096 | 51,510
Nonregulated: | Construction services: |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 1,072,663 | 907,767 | 1,062,055
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 609 | 18,660 | 57,474
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | 33,945 | 23,762 | 54,432
Nonregulated: | Other |  |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
External operating revenues |  |  |  |  |  |  |  |  |  | 1,413 | 1,436 | 1,532
Intersegment operating revenues |  |  |  |  |  |  |  |  |  | 7,230 | 7,755 | 7,832
Earnings (loss) on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  | $ (3,231) | $ (14,941) | $ (7,386)
[1] | Includes eliminations for the presentation of income tax adjustments between continuing and discontinued operations.
[2] | Capital expenditures for 2016, 2015 and 2014 include noncash capital expenditure-related accounts payable and AFUDC, totaling $(15.8) million, $35.3 million and $5.1 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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