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Goodwill rollforward (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Goodwill [Roll Forward] |  |  |  |  |
Balance at beginning of period | $ 631,791 | $ 635,204 | [1] | $ 635,204 | [1]
Goodwill acquired during the year | 0 | 6,323 |  | 6,324 |
Held for sale |  |  |  | (9,737) |
Balance at end of period | 631,791 | 641,527 | [1] | 631,791 |
Natural gas distribution [Member] |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |
Balance at beginning of period | 345,736 | 345,736 |  | 345,736 |
Goodwill acquired during the year | 0 | 0 |  | 0 |
Held for sale |  |  |  | 0 |
Balance at end of period | 345,736 | 345,736 |  | 345,736 |
Pipeline and midstream [Member] |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |
Balance at beginning of period | 0 | 9,737 | [1] | 9,737 | [1]
Goodwill acquired during the year |  | 0 |  | 0 |
Held for sale |  |  |  | (9,737) |
Balance at end of period |  | 9,737 | [1] | 0 |
Accumulated impairment which occurred in prior periods |  | 12,300 |  | 12,300 |
Construction materials and contracting [Member] |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |
Balance at beginning of period | 176,290 | 176,290 |  | 176,290 |
Goodwill acquired during the year | 0 | 0 |  | 0 |
Held for sale |  |  |  | 0 |
Balance at end of period | 176,290 | 176,290 |  | 176,290 |
Construction services [Member] |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |
Balance at beginning of period | 109,765 | 103,441 |  | 103,441 |
Goodwill acquired during the year | 0 | 6,323 |  | 6,324 |
Held for sale |  |  |  | 0 |
Balance at end of period | $ 109,765 | $ 109,764 |  | $ 109,765 |
[1] | Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Amount of accumulated impairment loss for 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a,h) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Amount of divestiture 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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