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Business segment data (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,378,213 | $ 1,563,799 | $ 1,303,573 | $ 1,091,191 | $ 1,209,875 | $ 1,280,787 | $ 1,064,597 | $ 976,293 | $ 5,336,776 | $ 4,531,552 | $ 4,443,351
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 256,017 | 220,205 | 207,486
Operating income (loss): | 130,221 | 189,470 | 97,311 | 64,218 | 118,351 | 140,004 | 73,992 | 69,376 | 481,220 | 401,723 | 424,048
Interest expense |  |  |  |  |  |  |  |  | 98,587 | 84,614 | 82,788
Income taxes |  |  |  |  |  |  |  |  | 63,279 | 47,485 | 65,041
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 335,166 | 269,386 | 284,215
Income (loss) from discontinued operations, net of tax (a) | 261 | $ 1,509 | $ (1,320) | $ (163) | 2,846 | $ (118) | $ (273) | $ 477 | 287 | 2,932 | (3,783)
Earnings on common stock |  |  |  |  |  |  |  |  | 335,453 | 272,318 | 280,432
Capital expenditures: |  |  |  |  |  |  |  |  | 636,498 | 769,303 | 351,924
Total assets | 7,683,059 |  |  |  | 6,988,110 |  |  |  | 7,683,059 | 6,988,110 | 6,334,666
Property, plant and equipment | 7,908,628 |  |  |  | 7,397,321 |  |  |  | 7,908,628 | 7,397,321 |
Less accumulated depreciation, depletion and amortization | 2,991,486 |  |  |  | 2,818,644 |  |  |  | 2,991,486 | 2,818,644 | 2,691,641
Net property, plant and equipment | 4,917,142 |  |  |  | 4,578,677 |  |  |  | 4,917,142 | 4,578,677 | 4,079,188
Additional information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net noncash acquisitions, capital expenditure-related accounts payable and AFUDC |  |  |  |  |  |  |  |  | 4,800 | 33,400 | 10,500
Electric |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 351,725 | 335,123 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 58,721 | 50,982 | 47,715
Operating income (loss): |  |  |  |  |  |  |  |  | 64,039 | 65,148 | 79,902
Interest expense |  |  |  |  |  |  |  |  | 25,334 | 25,860 | 25,377
Income taxes |  |  |  |  |  |  |  |  | (12,650) | (6,482) | 7,699
Capital expenditures: |  |  |  |  |  |  |  |  | 99,449 | 186,105 | 109,107
Total assets | 1,680,194 |  |  |  | 1,613,822 |  |  |  | 1,680,194 | 1,613,822 | 1,470,922
Property, plant and equipment | 2,227,145 |  |  |  | 2,148,569 |  |  |  | 2,227,145 | 2,148,569 | 1,982,264
Natural gas distribution |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 865,222 | 823,247 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 79,564 | 72,486 | 69,381
Operating income (loss): |  |  |  |  |  |  |  |  | 69,188 | 72,336 | 84,239
Interest expense |  |  |  |  |  |  |  |  | 35,488 | 30,768 | 31,234
Income taxes |  |  |  |  |  |  |  |  | 1,405 | 4,075 | 22,756
Capital expenditures: |  |  |  |  |  |  |  |  | 206,799 | 205,896 | 146,981
Total assets | 2,574,965 |  |  |  | 2,375,871 |  |  |  | 2,574,965 | 2,375,871 | 2,201,081
Property, plant and equipment | 2,688,123 |  |  |  | 2,499,093 |  |  |  | 2,688,123 | 2,499,093 | 2,319,845
Pipeline and midstream |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 84,192 | 78,018 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 21,220 | 17,896 | 16,788
Operating income (loss): |  |  |  |  |  |  |  |  | 42,796 | 36,128 | 36,004
Interest expense |  |  |  |  |  |  |  |  | 7,198 | 5,964 | 4,990
Income taxes |  |  |  |  |  |  |  |  | 7,219 | 2,677 | 12,281
Capital expenditures: |  |  |  |  |  |  |  |  | 71,477 | 70,057 | 31,054
Total assets | 677,482 |  |  |  | 616,959 |  |  |  | 677,482 | 616,959 | 566,295
Property, plant and equipment | 834,215 |  |  |  | 764,959 |  |  |  | 834,215 | 764,959 | 700,284
Construction materials and contracting |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 2,189,651 | 1,925,185 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 77,450 | 61,158 | 55,862
Operating income (loss): |  |  |  |  |  |  |  |  | 179,955 | 141,426 | 143,230
Interest expense |  |  |  |  |  |  |  |  | 23,792 | 17,290 | 14,778
Income taxes |  |  |  |  |  |  |  |  | 37,389 | 28,357 | 5,405
Capital expenditures: |  |  |  |  |  |  |  |  | 190,092 | 280,396 | 44,302
Total assets | 1,684,161 |  |  |  | 1,508,032 |  |  |  | 1,684,161 | 1,508,032 | 1,238,696
Property, plant and equipment | 1,910,562 |  |  |  | 1,768,006 |  |  |  | 1,910,562 | 1,768,006 | 1,560,048
Construction services |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,845,896 | 1,369,772 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 17,038 | 15,728 | 15,739
Operating income (loss): |  |  |  |  |  |  |  |  | 126,426 | 86,764 | 81,292
Interest expense |  |  |  |  |  |  |  |  | 5,331 | 3,551 | 3,742
Income taxes |  |  |  |  |  |  |  |  | 29,973 | 20,000 | 25,558
Capital expenditures: |  |  |  |  |  |  |  |  | 60,500 | 25,081 | 18,630
Total assets | 761,127 |  |  |  | 604,798 |  |  |  | 761,127 | 604,798 | 591,382
Property, plant and equipment | 213,370 |  |  |  | 188,586 |  |  |  | 213,370 | 188,586 | 177,265
Other |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 90 | 207 |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 2,024 | 1,955 | 2,001
Operating income (loss): |  |  |  |  |  |  |  |  | (1,184) | (79) | (619)
Interest expense |  |  |  |  |  |  |  |  | 1,859 | 2,762 | 3,564
Income taxes |  |  |  |  |  |  |  |  | (57) | (1,142) | (1,809)
Capital expenditures: |  |  |  |  |  |  |  |  | 8,181 | 1,768 | 1,850
Total assets | 303,279 |  |  |  | 266,111 |  |  |  | 303,279 | 266,111 | 261,419
Property, plant and equipment | 35,213 |  |  |  | 28,108 |  |  |  | 35,213 | 28,108 | 31,123
Assets held for sale |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total assets | $ 1,851 |  |  |  | $ 2,517 |  |  |  | 1,851 | 2,517 | 4,871
Intersegment eliminations |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Intersegment operating revenues: |  |  |  |  |  |  |  |  | (77,149) | (64,307) | (58,060)
Interest expense |  |  |  |  |  |  |  |  | (415) | (1,581) | (897)
Income taxes |  |  |  |  |  |  |  |  | 0 | 0 | (6,849)
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 6,849
Regulated: |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,279,304 | 1,213,227 | 1,244,759
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 56,037 | 50,580 | 48,867
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 122,535 | 111,637 | 102,211
Regulated: | Electric |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 351,725 | 335,123 | 342,805
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 54,763 | 47,000 | 49,366
Regulated: | Natural gas distribution |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 865,222 | 823,247 | 848,388
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 0 | 0 | 0
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 39,517 | 37,732 | 32,225
Regulated: | Pipeline and midstream |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 62,357 | 54,857 | 53,566
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 56,037 | 50,580 | 48,867
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 28,255 | 26,905 | 20,620
Nonregulated: |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 4,057,472 | 3,318,325 | 3,198,592
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 21,112 | 13,727 | 9,193
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 212,631 | 157,749 | 175,155
Nonregulated: | Pipeline and midstream |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 21,835 | 23,161 | 19,602
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 215 | 325 | 178
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 1,348 | 1,554 | (127)
Nonregulated: | Construction materials and contracting |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 2,189,651 | 1,925,185 | 1,811,964
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 1,066 | 669 | 565
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 120,371 | 92,647 | 123,398
Nonregulated: | Construction services |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,845,896 | 1,369,772 | 1,366,317
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 3,370 | 1,681 | 1,285
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 92,998 | 64,309 | 53,306
Nonregulated: | Other |  |  |  |  |  |  |  |  |  |  |
Business segment data [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 90 | 207 | 709
Intersegment operating revenues: |  |  |  |  |  |  |  |  | 16,461 | 11,052 | 7,165
Earnings on common stock before income (loss) from discontinued operations |  |  |  |  |  |  |  |  | $ (2,086) | $ (761) | $ (1,422)

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