Goodwill rollforward (Details) - USD ($) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	$ 635,204	$ 636,039	$ 636,039
Goodwill acquired during the year		0	0
Goodwill acquired during the year/other				(835)
Balance as of end of period	[1]	635,204	636,039	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
Goodwill acquired during the year/other				0
Balance as of end of period		345,736	345,736	345,736
Pipeline and energy services [Member]
Goodwill [Roll Forward]
Balance as of beginning of period	[1]	9,737	9,737	9,737
Goodwill acquired during the year		0	0
Goodwill acquired during the year/other				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
Goodwill acquired during the year/other				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	104,276	104,276
Goodwill acquired during the year		0	0
Goodwill acquired during the year/other				(835)
Balance as of end of period		$ 103,441	$ 104,276	$ 103,441

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