Goodwill rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Goodwill [Roll Forward]
Balance at beginning of period	$ 631,791	$ 631,791	$ 631,791
Goodwill acquired during the year	0	0	0
Balance at end of period	631,791	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	0	0	0
Balance at end of period	176,290	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