Disaggregation of revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Operating revenues	$ 5,680,733	$ 5,532,750	$ 5,336,776
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(77,606)	(76,969)	(77,149)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(501,044)	(550,815)	(525,749)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues	162,409	157,232	147,114
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues		4,865	9,164
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues	14,680	14,918	11,708
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues	1,017,471	1,069,665	1,088,633
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues	1,712,503	1,659,152	1,627,833
Electrical & mechanical specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues	1,324,419	1,397,124	1,266,196
Transmission & distribution specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues	677,074	649,486	531,882
Other
Disaggregation of Revenue [Line Items]
Operating revenues	81,407	68,865	82,669
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	5,628,488	5,471,545	5,281,621
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	52,245	61,205	55,155
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	671,562	599,051	608,821
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	465,841	409,350	438,431
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	72,721	62,987	64,660
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	7,051	6,634	7,408
Electric
Disaggregation of Revenue [Line Items]
Operating revenues	349,039	331,538	351,725
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(543)	(491)	0
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues	42,902	32,452	35,574
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	355,564	329,479	347,712
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	(6,525)	2,059	4,013
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	126,841	122,663	125,369
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	137,556	131,477	141,596
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	41,757	36,744	37,765
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	7,051	6,634	7,408
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues	971,364	847,651	865,222
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(576)	(534)	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues	48,408	45,546	45,449
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues	10,567	10,753	12,726
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	962,369	836,269	865,357
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	8,995	11,382	(135)
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	544,721	476,388	483,452
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	328,285	277,873	296,835
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	30,964	26,243	26,895
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Operating revenues	82,858	85,346	84,192
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(59,678)	(58,531)	(56,252)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues	114,001	111,686	101,665
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues		4,865	9,164
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues	14,680	14,918	11,708
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues	13,667	12,216	17,687
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	82,670	85,154	83,972
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	188	192	220
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues	2,228,306	2,177,585	2,189,651
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(624)	(417)	(1,066)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(501,044)	(550,815)	(525,749)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues	1,017,471	1,069,665	1,088,633
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues	1,712,503	1,659,152	1,627,833
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	2,228,306	2,177,585	2,189,651
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	0	0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues	2,049,082	2,090,685	1,845,896
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(2,555)	(5,038)	(3,370)
Construction services | Electrical & mechanical specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues	1,324,419	1,397,124	1,266,196
Construction services | Transmission & distribution specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues	677,074	649,486	531,882
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues	557	1,541	131
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	1,999,495	2,043,113	1,794,839
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	49,587	47,572	51,057
Other
Disaggregation of Revenue [Line Items]
Operating revenues	84	(55)	90
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(13,630)	(11,958)	(16,461)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues	13,714	11,903	16,551
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues	84	(55)	90
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues	$ 0	$ 0	$ 0