Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,303,573	$ 1,064,597	$ 2,394,764	$ 2,040,890
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(11,281)	(9,981)	(43,285)	(34,490)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(145,925)	(125,567)	(181,066)	(159,837)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	35,510	31,371	72,138	64,368
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,396	2,310	4,517	4,580
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,623	2,634	5,269	5,768
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	297,124	247,558	380,164	321,622
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	444,768	387,632	624,077	561,223
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	319,276	216,371	618,805	450,192
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	133,288	95,261	240,686	182,442
Other
Disaggregation of Revenue [Line Items]
Revenues	20,545	19,225	44,136	37,412
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,287,052	1,051,077	2,368,092	2,015,619
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	16,521	13,520	26,672	25,271
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	97,447	95,440	334,612	323,509
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	74,481	73,496	231,945	225,088
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	14,921	13,453	32,416	30,032
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,879	1,874	3,678	3,710
Electric
Disaggregation of Revenue [Line Items]
Revenues	81,011	78,499	173,578	165,904
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	8,417	8,425	17,538	16,678
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	79,308	77,953	171,339	166,696
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,703	546	2,239	(792)
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	26,437	26,752	62,993	61,935
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	33,231	32,676	68,902	67,377
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	9,344	8,226	18,228	16,996
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	1,879	1,874	3,678	3,710
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	133,867	129,540	476,014	462,204
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	10,706	10,084	22,276	21,263
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	2,923	3,614	6,836	7,613
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	131,466	128,433	477,962	461,197
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	2,401	1,107	(1,948)	1,007
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	71,010	68,688	271,619	261,574
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	41,250	40,820	163,043	157,711
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	5,577	5,227	14,188	13,036
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Revenues	28,680	24,060	37,293	32,893
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(7,513)	(6,539)	(31,468)	(28,298)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	24,804	21,287	49,862	43,105
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,396	2,310	4,517	4,580
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,623	2,634	5,269	5,768
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Revenues	6,293	4,326	8,989	7,652
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	28,603	24,018	37,169	32,807
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	77	42	124	86
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	595,799	509,388	822,911	722,672
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(168)	(235)	(264)	(336)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(145,925)	(125,567)	(181,066)	(159,837)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	297,124	247,558	380,164	321,622
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	444,768	387,632	624,077	561,223
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	595,799	509,388	822,911	722,672
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	464,192	323,020	884,925	657,071
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(721)	(540)	(849)	(550)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	319,276	216,371	618,805	450,192
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	133,288	95,261	240,686	182,442
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	9	103	26	17
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	451,852	311,195	858,668	632,101
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	12,340	11,825	26,257	24,970
Other
Disaggregation of Revenue [Line Items]
Revenues	24	90	43	146
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(2,879)	(2,667)	(10,704)	(5,306)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	2,903	2,757	10,747	5,452
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	24	90	43	146
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0