Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,197,373	$ 1,091,191
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(31,085)	(32,003)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(44,104)	(35,140)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	39,230	36,628
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,090	2,121
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,046	2,646
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	98,401	83,039
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	207,910	179,309
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	372,209	299,530
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	130,150	107,398
Other
Disaggregation of Revenue [Line Items]
Revenues	17,531	23,590
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,183,547	1,081,040
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	13,826	10,151
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	219,050	237,164
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	148,583	157,464
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	18,888	17,495
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,648	1,799
Electric
Disaggregation of Revenue [Line Items]
Revenues	85,908	92,566
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(195)	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	8,450	9,121
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	86,206	92,030
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	(298)	536
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	32,349	36,555
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	33,587	35,671
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	10,367	8,884
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,648	1,799
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	326,443	342,147
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(185)	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	11,798	11,570
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	2,498	3,913
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	324,329	346,496
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	2,114	(4,349)
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	186,701	200,609
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	114,996	121,793
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	8,521	8,611
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Revenues	10,655	8,613
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(25,199)	(23,955)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	27,432	25,058
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,090	2,121
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,046	2,646
Pipeline | Other
Disaggregation of Revenue [Line Items]
Revenues	3,241	2,696
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	10,610	8,566
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	45	47
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	262,144	227,113
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(63)	(95)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(44,104)	(35,140)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	98,401	83,039
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	207,910	179,309
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	262,144	227,113
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	0	0
Construction services
Disaggregation of Revenue [Line Items]
Revenues	512,205	420,733
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(2,470)	(129)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	372,209	299,530
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	130,150	107,398
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	351	17
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	500,240	406,816
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	11,965	13,917
Other
Disaggregation of Revenue [Line Items]
Revenues	18	19
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(2,973)	(7,824)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	2,991	7,843
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	18	19
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0