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Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Goodwill [Roll Forward]
Balance at beginning of period	$ 714,963	$ 681,358	$ 681,358
Goodwill acquired during the year	2,900	30,919	33,214
Measurement period adjustments	(217)	400	391
Balance at end of period	717,646	712,677	714,963
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
Measurement period adjustments	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	226,003	217,234	217,234
Goodwill acquired during the year	2,900	6,483	8,778
Measurement period adjustments	(217)	0	(9)
Balance at end of period	228,686	223,717	226,003
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	143,224	118,388	118,388
Goodwill acquired during the year	0	24,436	24,436
Measurement period adjustments	0	400	400
Balance at end of period	$ 143,224	$ 143,224	$ 143,224

X

- 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

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-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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X

- 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

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X

- 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

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X

- 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.

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