Goodwill rollforward (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	$ 635,204	$ 635,204	$ 635,204
Goodwill acquired during the year		6,323	0	0
Balance as of end of period	[1]	641,527	635,204	635,204
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance as of beginning of period		345,736	345,736	345,736
Goodwill acquired during the year		0	0	0
Balance as of end of period		345,736	345,736	345,736
Pipeline and midstream [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	9,737	9,737	9,737
Goodwill acquired during the year		0	0	0
Balance as of end of period	[1]	9,737	9,737	9,737
Goodwill, impaired, accumulated impairment loss		12,300	12,300	12,300
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance as of beginning of period		176,290	176,290	176,290
Goodwill acquired during the year		0	0	0
Balance as of end of period		176,290	176,290	176,290
Construction services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period		103,441	103,441	103,441
Goodwill acquired during the year		6,323	0	0
Balance as of end of period		$ 109,764	$ 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.