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Goodwill rollforward (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Goodwill [Roll Forward] |  |  |
Balance at beginning of period | $ 631,791 | $ 631,791 | $ 631,791
Goodwill acquired during the year | 8,412 | 0 | 0
Balance at end of period | 640,203 | 631,791 | 631,791
Natural gas distribution |  |  |
Goodwill [Roll Forward] |  |  |
Balance at beginning of period | 345,736 | 345,736 | 345,736
Goodwill acquired during the year | 0 | 0 | 0
Balance at end of period | 345,736 | 345,736 | 345,736
Construction materials and contracting |  |  |
Goodwill [Roll Forward] |  |  |
Balance at beginning of period | 176,290 | 176,290 | 176,290
Goodwill acquired during the year | 8,412 | 0 | 0
Balance at end of period | 184,702 | 176,290 | 176,290
Construction services |  |  |
Goodwill [Roll Forward] |  |  |
Balance at beginning of period | 109,765 | 109,765 | 109,765
Goodwill acquired during the year | 0 | 0 | 0
Balance at end of period | $ 109,765 | $ 109,765 | $ 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 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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- Details
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