Goodwill and other intangible assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	$ 635,204	$ 636,039
Goodwill acquired during the year		0
Goodwill acquired during the year/other			(835)
Balance as of end of period	[1]	635,204	635,204
Natural gas distribution:
Goodwill [Roll Forward]
Balance as of beginning of period		345,736	345,736
Goodwill acquired during the year		0
Goodwill acquired during the year/other			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
Goodwill acquired during the year/other			0
Balance as of end of period	[1]	9,737	9,737
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
Goodwill acquired during the year/other			0
Balance as of end of period		176,290	176,290
Construction services:
Goodwill [Roll Forward]
Balance as of beginning of period		103,441	104,276
Goodwill acquired during the year		0
Goodwill acquired during the year/other			(835)
Balance as of end of period		$ 103,441	$ 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.