Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 1,416,559	$ 1,227,938
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(31,730)	(30,935)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(46,033)	(46,716)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	43,955	41,869
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,719	4,029
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	114,267	96,025
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	241,732	216,412
Electrical & mechanical specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	392,808	355,190
Transmission & distribution specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	148,466	151,363
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	21,460	18,819
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	1,409,053	1,216,016
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	7,506	11,922
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	296,120	236,573
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	199,209	152,980
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	23,330	18,841
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,750	1,566
Electric
Disaggregation of Revenue [Line Items]
Operating revenues:	93,594	84,673
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(124)	(136)
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	12,753	9,773
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	97,589	87,596
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,995)	(2,923)
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	37,304	33,436
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	35,600	32,928
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	10,306	10,029
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,750	1,566
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues:	450,418	350,206
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(136)	(142)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	12,381	12,452
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	2,609	3,009
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	450,303	347,320
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	115	2,886
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	258,816	203,137
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	163,609	120,052
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	13,024	8,812
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Operating revenues:	11,120	9,913
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(25,940)	(26,229)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	31,574	29,417
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,719	4,029
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	1,709	2,660
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	11,062	9,877
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	58	36
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	309,836	265,659
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(130)	(62)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(46,033)	(46,716)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	114,267	96,025
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	241,732	216,412
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	309,836	265,659
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues:	551,591	517,470
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(1,059)	(1,042)
Construction services | Electrical & mechanical specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	392,808	355,190
Construction services | Transmission & distribution specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	148,466	151,363
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	48	36
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	540,263	505,547
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	11,328	11,923
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	0	17
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(4,341)	(3,324)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	4,341	3,341
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	0	17
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 0	$ 0