Goodwill and other intangible assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Balance as of beginning of period	$ 631,791	$ 631,791
Goodwill, Acquired During Period	33,131	0
Balance as of end of period	664,922	631,791
Natural gas distribution
Goodwill [Roll Forward]
Balance as of beginning of period	345,736	345,736
Goodwill, Acquired During Period	0	0
Balance as of end of period	345,736	345,736
Construction materials and contracting
Goodwill [Roll Forward]
Balance as of beginning of period	176,290	176,290
Goodwill, Acquired During Period	33,131	0
Balance as of end of period	209,421	176,290
Construction services
Goodwill [Roll Forward]
Balance as of beginning of period	109,765	109,765
Goodwill, Acquired During Period	0	0
Balance as of end of period	109,765	$ 109,765
Series of Individually Immaterial Business Acquisitions [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 33,100