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Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 1,423,657	$ 1,362,928	$ 2,651,595	$ 2,560,302
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(12,440)	(12,222)	(43,375)	(43,306)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(145,780)	(164,719)	(192,496)	(208,823)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	39,136	38,315	81,005	77,545
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		1,181		3,271
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,094	3,104	7,123	6,150
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	280,834	303,356	376,859	401,757
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	498,762	482,498	715,174	690,408
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	347,702	324,921	702,892	697,130
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	165,123	160,696	316,486	290,846
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	20,515	17,064	39,334	34,595
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	1,409,489	1,347,026	2,625,505	2,530,574
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	14,168	15,902	26,090	29,728
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	112,738	106,607	349,311	325,657
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	81,446	70,904	234,426	219,487
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	16,534	13,730	35,375	32,618
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,825	1,591	3,391	3,239
Electric
Disaggregation of Revenue [Line Items]
Operating revenues:	83,650	76,438	168,324	162,346
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(136)	(196)	(271)	(391)
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	9,465	7,717	19,238	16,167
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	84,600	75,015	172,197	161,221
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	(950)	1,423	(3,873)	1,125
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	29,258	27,954	62,694	60,303
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	33,834	29,877	66,762	63,464
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	10,354	8,072	20,383	18,439
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	1,825	1,591	3,391	3,239
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues:	153,788	141,494	503,994	467,937
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(142)	(185)	(284)	(370)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	11,451	10,350	23,903	22,148
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	2,794	2,711	5,803	5,209
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	151,375	138,214	498,695	462,543
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	2,413	3,280	5,299	5,394
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	83,480	78,653	286,617	265,354
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	47,612	41,027	167,664	156,023
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues:	6,180	5,658	14,992	14,179
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:	27,773	27,658	37,685	38,313
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(7,862)	(7,999)	(34,092)	(33,198)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues:	27,685	27,965	57,102	55,397
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues:		1,181		3,271
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues:	3,094	3,104	7,123	6,150
Pipeline | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	4,810	3,363	7,470	6,604
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	27,727	27,614	37,603	38,224
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	46	44	82	89
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	633,674	621,045	899,333	883,190
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(142)	(90)	(204)	(152)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(145,780)	(164,719)	(192,496)	(208,823)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues:	280,834	303,356	376,859	401,757
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues:	498,762	482,498	715,174	690,408
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:	633,674	621,045	899,333	883,190
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:	524,744	496,405	1,042,214	1,008,610
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(797)	(779)	(1,839)	(3,249)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	347,702	324,921	702,892	697,130
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues:	165,123	160,696	316,486	290,846
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	57	412	93	763
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	512,085	485,250	1,017,632	985,490
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	12,659	11,155	24,582	23,120
Other
Disaggregation of Revenue [Line Items]
Operating revenues:	28	(112)	45	(94)
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues:	(3,361)	(2,973)	(6,685)	(5,946)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues:	3,389	2,861	6,730	5,852
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues:	28	(112)	45	(94)
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues:	$ 0	$ 0	$ 0	$ 0

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