Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,587,289	$ 1,563,799	$ 4,147,591	$ 3,958,563
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(7,543)	(8,043)	(50,849)	(51,328)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(234,693)	(231,078)	(443,516)	(412,144)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	38,742	36,639	116,287	108,777
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	973	2,510	4,244	7,027
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,885	3,044	10,035	8,313
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	448,569	461,716	850,326	841,881
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	608,673	638,862	1,299,081	1,262,938
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	352,845	317,202	1,049,975	936,008
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	187,202	151,285	478,047	391,971
Other
Disaggregation of Revenue [Line Items]
Revenues	16,972	20,551	51,568	64,686
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,576,190	1,549,301	4,106,764	3,917,392
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	11,099	14,498	40,827	41,171
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	81,485	75,278	407,142	409,889
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	64,419	65,818	283,906	297,763
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	12,882	13,655	45,500	46,071
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,779	1,862	5,018	5,540
Electric
Disaggregation of Revenue [Line Items]
Revenues	87,447	89,845	249,793	263,423
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(195)	0	(586)	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	7,663	9,380	23,830	26,918
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	86,449	87,636	247,670	258,974
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	998	2,209	2,123	4,449
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	33,086	30,376	93,389	93,368
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	35,688	36,670	99,152	105,572
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	8,428	9,348	26,867	27,576
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,779	1,862	5,018	5,540
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	94,703	93,642	562,641	569,656
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(185)	0	(555)	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	11,159	11,410	33,307	33,686
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	2,590	2,708	7,800	9,544
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	95,148	92,475	557,692	570,437
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	(445)	1,167	4,949	(781)
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	48,399	44,902	313,753	316,521
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	28,731	29,148	184,754	192,191
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	4,454	4,307	18,633	18,495
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Revenues	32,068	32,533	70,381	69,827
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,622)	(3,831)	(36,820)	(35,298)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	27,583	25,229	82,980	75,091
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	973	2,510	4,244	7,027
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,885	3,044	10,035	8,313
Pipeline | Other
Disaggregation of Revenue [Line Items]
Revenues	3,203	5,534	9,807	14,523
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	32,022	32,486	70,246	69,656
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	46	47	135	171
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	822,439	869,376	1,705,629	1,692,287
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(110)	(124)	(262)	(388)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(234,693)	(231,078)	(443,516)	(412,144)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	448,569	461,716	850,326	841,881
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	608,673	638,862	1,299,081	1,262,938
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	822,439	869,376	1,705,629	1,692,287
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Construction services
Disaggregation of Revenue [Line Items]
Revenues	550,608	478,381	1,559,217	1,363,305
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(425)	(1,226)	(3,674)	(2,076)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	352,845	317,202	1,049,975	936,008
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	187,202	151,285	478,047	391,971
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	486	45	1,249	70
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	540,108	467,306	1,525,597	1,325,973
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	10,500	11,075	33,620	37,332
Other
Disaggregation of Revenue [Line Items]
Revenues	24	22	(70)	65
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,006)	(2,862)	(8,952)	(13,566)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	3,030	2,884	8,882	13,631
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	24	22	(70)	65
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0