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Goodwill rollforward (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | [1] | $ 635,204 | $ 635,204 | $ 635,204
Goodwill acquired during the year |  | 6,323 | 0 | 0
Balance as of end of period | [1] | 641,527 | 635,204 | 635,204
Natural gas distribution [Member] |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period |  | 345,736 | 345,736 | 345,736
Goodwill acquired during the year |  | 0 | 0 | 0
Balance as of end of period |  | 345,736 | 345,736 | 345,736
Pipeline and midstream [Member] |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | [1] | 9,737 | 9,737 | 9,737
Goodwill acquired during the year |  | 0 | 0 | 0
Balance as of end of period | [1] | 9,737 | 9,737 | 9,737
Goodwill, impaired, accumulated impairment loss |  | 12,300 | 12,300 | 12,300
Construction materials and contracting [Member] |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period |  | 176,290 | 176,290 | 176,290
Goodwill acquired during the year |  | 0 | 0 | 0
Balance as of end of period |  | 176,290 | 176,290 | 176,290
Construction services [Member] |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period |  | 103,441 | 103,441 | 103,441
Goodwill acquired during the year |  | 6,323 | 0 | 0
Balance as of end of period |  | $ 109,764 | $ 103,441 | $ 103,441
[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=64834752&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 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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