Goodwill rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Goodwill [Roll Forward]
Balance at beginning of period	$ 681,358	$ 664,922	$ 664,922
Goodwill acquired during the year	32,169	14,473	23,105
Measurement period adjustments	0	0	(6,669)
Balance at end of period	713,527	679,395	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	0	(6,669)
Balance at end of period	223,717	223,894	217,234
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	118,388	109,765	109,765
Goodwill acquired during the year	25,686	0	8,623
Measurement period adjustments	0	0	0
Balance at end of period	$ 144,074	$ 109,765	$ 118,388