Goodwill and other intangible assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Goodwill [Roll Forward]
Balance as of beginning of period	$ 631,791	$ 635,204	[1]
Goodwill acquired during the year	0	6,324
Held for sale		9,737
Balance as of end of period	631,791	631,791
Natural gas distribution:
Goodwill [Roll Forward]
Balance as of beginning of period	345,736	345,736
Goodwill acquired during the year	0	0
Held for sale		0
Balance as of end of period	345,736	345,736
Pipeline and midstream:
Goodwill [Roll Forward]
Balance as of beginning of period	0	9,737	[1]
Goodwill acquired during the year		0
Held for sale		9,737
Balance as of end of period		0
Accumulated impairment which occurred in prior periods				$ 12,300
Construction materials and contracting:
Goodwill [Roll Forward]
Balance as of beginning of period	176,290	176,290
Goodwill acquired during the year	0	0
Held for sale		0
Balance as of end of period	176,290	176,290
Construction services:
Goodwill [Roll Forward]
Balance as of beginning of period	109,765	103,441
Goodwill acquired during the year	0	6,324
Held for sale		0
Balance as of end of period	$ 109,765	$ 109,765

[1] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and midstream segment, which occurred in prior periods.