Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 1,362,928	$ 1,303,573	$ 2,560,302	$ 2,394,764
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(12,222)	(11,281)	(43,306)	(43,285)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(164,719)	(145,925)	(208,823)	(181,066)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	38,315	35,510	77,545	72,138
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	1,181	2,396	3,271	4,517
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,104	2,623	6,150	5,269
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	303,356	297,124	401,757	380,164
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	482,498	444,768	690,408	624,077
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	324,921	319,276	697,130	618,805
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	160,696	133,288	290,846	240,686
Other
Disaggregation of Revenue [Line Items]
Revenues	17,064	20,545	34,595	44,136
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,347,026	1,287,052	2,530,574	2,368,092
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	15,902	16,521	29,728	26,672
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	106,607	97,447	325,657	334,612
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	70,904	74,481	219,487	231,945
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	13,730	14,921	32,618	32,416
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,591	1,879	3,239	3,678
Electric
Disaggregation of Revenue [Line Items]
Revenues	76,438	81,011	162,346	173,578
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(196)	0	(391)	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	7,717	8,417	16,167	17,538
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	75,015	79,308	161,221	171,339
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,423	1,703	1,125	2,239
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	27,954	26,437	60,303	62,993
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	29,877	33,231	63,464	68,902
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	8,072	9,344	18,439	18,228
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,591	1,879	3,239	3,678
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	141,494	133,867	467,937	476,014
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(185)	0	(370)	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	10,350	10,706	22,148	22,276
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	2,711	2,923	5,209	6,836
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	138,214	131,466	462,543	477,962
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	3,280	2,401	5,394	(1,948)
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	78,653	71,010	265,354	271,619
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	41,027	41,250	156,023	163,043
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	5,658	5,577	14,179	14,188
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Pipeline
Disaggregation of Revenue [Line Items]
Revenues	27,658	28,680	38,313	37,293
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(7,999)	(7,513)	(33,198)	(31,468)
Pipeline | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	27,965	24,804	55,397	49,862
Pipeline | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	1,181	2,396	3,271	4,517
Pipeline | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	3,104	2,623	6,150	5,269
Pipeline | Other
Disaggregation of Revenue [Line Items]
Revenues	3,363	6,293	6,604	8,989
Pipeline | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	27,614	28,603	38,224	37,169
Pipeline | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	44	77	89	124
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	621,045	595,799	883,190	822,911
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(90)	(168)	(152)	(264)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(164,719)	(145,925)	(208,823)	(181,066)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	303,356	297,124	401,757	380,164
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	482,498	444,768	690,408	624,077
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	621,045	595,799	883,190	822,911
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	496,405	464,192	1,008,610	884,925
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(779)	(721)	(3,249)	(849)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	324,921	319,276	697,130	618,805
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	160,696	133,288	290,846	240,686
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	412	9	763	26
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	485,250	451,852	985,490	858,668
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	11,155	12,340	23,120	26,257
Other
Disaggregation of Revenue [Line Items]
Revenues	(112)	24	(94)	43
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(2,973)	(2,879)	(5,946)	(10,704)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	2,861	2,903	5,852	10,747
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	(112)	24	(94)	43
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0