Goodwill rollforward (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Goodwill [Roll Forward]
Balance at beginning of period	$ 664,922	$ 631,791	$ 631,791
Goodwill acquired during the year	23,096	8,412	33,131
Measurement period adjustments	(6,669)	0	0
Balance at end of period	681,349	640,203	664,922
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	209,421	176,290	176,290
Goodwill acquired during the year	14,473	8,412	33,131
Measurement period adjustments	(6,669)	0	0
Balance at end of period	217,225	184,702	209,421
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	109,765	109,765	109,765
Goodwill acquired during the year	8,623	0	0
Measurement period adjustments	0	0	0
Balance at end of period	$ 118,388	$ 109,765	$ 109,765