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Goodwill rollforward (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Goodwill [Roll Forward] |  |
Balance at beginning of period | $ 681,358,000 | $ 664,922,000
Goodwill acquired during the year | 33,214,000 | 23,105,000
Goodwill, Purchase Accounting Adjustments | 391,000 | (6,669,000)
Balance at end of period | 714,963,000 | 681,358,000
Natural gas distribution |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 345,736,000 | 345,736,000
Goodwill acquired during the year | 0 | 0
Goodwill, Purchase Accounting Adjustments | 0 | 0
Balance at end of period | 345,736,000 | 345,736,000
Construction materials and contracting |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 217,234,000 | 209,421,000
Goodwill acquired during the year | 8,778,000 | 14,482,000
Goodwill, Purchase Accounting Adjustments | (9,000) | (6,669,000)
Balance at end of period | 226,003,000 | 217,234,000
Construction services |  |
Goodwill [Roll Forward] |  |
Balance at beginning of period | 118,388,000 | 109,765,000
Goodwill acquired during the year | 24,436,000 | 8,623,000
Goodwill, Purchase Accounting Adjustments | 400,000 | 0
Balance at end of period | $ 143,224,000 | $ 118,388,000

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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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: 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=120320667&loc=SL49117168-202975 Reference 3: 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=121556970&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=121556970&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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