Goodwill rollforward (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Balance at beginning of period	$ 681,358,000	$ 664,922,000
Goodwill acquired during the year	33,214,000	23,105,000
Goodwill, Purchase Accounting Adjustments	391,000	(6,669,000)
Balance at end of period	714,963,000	681,358,000
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736,000	345,736,000
Goodwill acquired during the year	0	0
Goodwill, Purchase Accounting Adjustments	0	0
Balance at end of period	345,736,000	345,736,000
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	217,234,000	209,421,000
Goodwill acquired during the year	8,778,000	14,482,000
Goodwill, Purchase Accounting Adjustments	(9,000)	(6,669,000)
Balance at end of period	226,003,000	217,234,000
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	118,388,000	109,765,000
Goodwill acquired during the year	24,436,000	8,623,000
Goodwill, Purchase Accounting Adjustments	400,000	0
Balance at end of period	$ 143,224,000	$ 118,388,000