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Goodwill rollforward (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Goodwill [Roll Forward] |  |
Balance at beginning of period | $ 664,922 | $ 631,791
Goodwill acquired during the year | 23,105 | 33,131
Goodwill | (6,669) | 0
Balance at end of period | 681,358 | 664,922
Natural gas distribution |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 345,736 | 345,736
Goodwill acquired during the year | 0 | 0
Goodwill | 0 | 0
Balance at end of period | 345,736 | 345,736
Construction materials and contracting |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 209,421 | 176,290
Goodwill acquired during the year | 14,482 | 33,131
Goodwill | (6,669) | 0
Balance at end of period | 217,234 | 209,421
Construction services |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 109,765 | 109,765
Goodwill acquired during the year | 8,623 | 0
Goodwill | 0 | 0
Balance at end of period | $ 118,388 | $ 109,765

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
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- Definition Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e961-128460 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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