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

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