Goodwill rollforward (Details) - USD ($) $ in Thousands	6 Months Ended			12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016		Dec. 31, 2016
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791	$ 635,204	[1]	$ 635,204	[1]
Goodwill acquired during the year	0	6,323		6,324
Held for sale				(9,737)
Balance at end of period	631,791	641,527	[1]	631,791
Natural gas distribution [Member]
Goodwill [Roll Forward]
Balance at beginning of period	345,736	345,736		345,736
Goodwill acquired during the year	0	0		0
Held for sale				0
Balance at end of period	345,736	345,736		345,736
Pipeline and midstream [Member]
Goodwill [Roll Forward]
Balance at beginning of period	0	9,737	[1]	9,737	[1]
Goodwill acquired during the year		0		0
Held for sale				(9,737)
Balance at end of period		9,737	[1]	0
Accumulated impairment which occurred in prior periods		12,300		12,300
Construction materials and contracting [Member]
Goodwill [Roll Forward]
Balance at beginning of period	176,290	176,290		176,290
Goodwill acquired during the year	0	0		0
Held for sale				0
Balance at end of period	176,290	176,290		176,290
Construction services [Member]
Goodwill [Roll Forward]
Balance at beginning of period	109,765	103,441		103,441
Goodwill acquired during the year	0	6,323		6,324
Held for sale				0
Balance at end of period	$ 109,765	$ 109,764		$ 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.