Goodwill rollforward (Details) - USD ($) $ in Thousands	6 Months Ended			12 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791		$ 631,791	$ 631,791
Goodwill acquired during the year	10,583	[1]	0	0
Balance at end of period	642,374		631,791	631,791
Natural gas distribution
Goodwill [Roll Forward]
Balance at beginning of period	345,736		345,736	345,736
Goodwill acquired during the year	0		0	0
Balance at end of period	345,736		345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance at beginning of period	176,290		176,290	176,290
Goodwill acquired during the year	10,583	[1]	0	0
Balance at end of period	186,873		176,290	176,290
Construction services
Goodwill [Roll Forward]
Balance at beginning of period	109,765		109,765	109,765
Goodwill acquired during the year	0		0	0
Balance at end of period	$ 109,765		$ 109,765	$ 109,765

[1] Relates to business acquisitions that occurred during the period, as discussed in Note 9.