Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Goodwill [Roll Forward]
Balance at beginning of period	$ 681,358	$ 664,922	$ 664,922
Goodwill acquired during the year	30,919	23,096	23,105
Measurement period adjustments	400	(6,669)	(6,669)
Balance at end of period	712,677	681,349	681,358
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	217,234	209,421	209,421
Goodwill acquired during the year	6,483	14,473	14,482
Measurement period adjustments	0	(6,669)	(6,669)
Balance at end of period	223,717	217,225	217,234
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	118,388	109,765	109,765
Goodwill acquired during the year	24,436	8,623	8,623
Measurement period adjustments	400	0	0
Balance at end of period	$ 143,224	$ 118,388	$ 118,388