Goodwill rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Balance at beginning of period	$ 664,922	$ 631,791
Goodwill acquired during the year	23,105	33,131
Goodwill	(6,669)	0
Balance at end of period	681,358	664,922
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736
Goodwill acquired during the year	0	0
Goodwill	0	0
Balance at end of period	345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	209,421	176,290
Goodwill acquired during the year	14,482	33,131
Goodwill	(6,669)	0
Balance at end of period	217,234	209,421
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	109,765	109,765
Goodwill acquired during the year	8,623	0
Goodwill	0	0
Balance at end of period	$ 118,388	$ 109,765