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Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 1,586,013	$ 1,587,289	$ 4,237,608	$ 4,147,591
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(7,983)	(7,543)	(51,356)	(50,849)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(201,630)	(234,693)	(394,126)	(443,516)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	39,870	38,742	120,875	116,287
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		973		4,244
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,483	3,885	10,606	10,035
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	415,106	448,569	791,964	850,326
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	617,823	608,673	1,332,997	1,299,081
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	317,238	352,845	1,020,130	1,049,975
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	185,842	187,202	502,328	478,047
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	21,487	16,972	60,820	51,568
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	1,575,186	1,576,190	4,200,691	4,106,764
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	10,827	11,099	36,917	40,827
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	91,696	81,485	441,007	407,142
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	74,371	64,419	308,797	283,906
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	15,890	12,882	51,265	45,500
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,993	1,779	5,384	5,018
Electric
Disaggregation of Revenue [Line Items]
Operating revenues:	98,930	87,447	267,254	249,793
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(136)	(195)	(407)	(586)
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	12,589	7,663	31,827	23,830
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	100,386	86,449	272,583	247,670
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	(1,456)	998	(5,329)	2,123
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	36,412	33,086	99,106	93,389
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	39,033	35,688	105,795	99,152
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	10,495	8,428	30,878	26,867
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,993	1,779	5,384	5,018
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues:	110,378	94,703	614,373	562,641
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(142)	(185)	(425)	(555)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	11,812	11,159	35,715	33,307
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	2,110	2,590	7,913	7,800
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	109,797	95,148	608,493	557,692
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	581	(445)	5,880	4,949
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	55,284	48,399	341,901	313,753
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	35,338	28,731	203,002	184,754
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	5,395	4,454	20,387	18,633
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Operating revenues:	30,968	32,068	68,653	70,381
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,908)	(3,622)	(38,000)	(36,820)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	28,058	27,583	85,160	82,980
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		973		4,244
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,483	3,885	10,606	10,035
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	3,290	3,203	10,760	9,807
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	30,923	32,022	68,526	70,246
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	45	46	127	135
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	831,189	822,439	1,730,521	1,705,629
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(110)	(110)	(314)	(262)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(201,630)	(234,693)	(394,126)	(443,516)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	415,106	448,569	791,964	850,326
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	617,823	608,673	1,332,997	1,299,081
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	831,189	822,439	1,730,521	1,705,629
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	0	0	0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues:	514,523	550,608	1,556,737	1,559,217
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(289)	(425)	(2,128)	(3,674)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	317,238	352,845	1,020,130	1,049,975
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	185,842	187,202	502,328	478,047
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	75	486	168	1,249
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	502,866	540,108	1,520,498	1,525,597
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	11,657	10,500	36,239	33,620
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	25	24	70	(70)
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,398)	(3,006)	(10,082)	(8,952)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	3,423	3,030	10,152	8,882
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	25	24	70	(70)
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 0	$ 0	$ 0	$ 0

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