Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,378,213	$ 1,563,799	$ 1,303,573	$ 1,091,191	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 5,336,776	$ 4,531,552	$ 4,443,351
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(77,149)	(64,307)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(525,749)	(465,969)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									147,114	132,397
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,164	9,159
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,708	11,543
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,088,633	968,755
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,627,833	1,423,068
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,266,196	926,875
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									531,882	392,544
Other
Disaggregation of Revenue [Line Items]
Operating revenues									82,669	76,796
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									5,281,621	4,469,761
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									55,155	61,791
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									608,821	579,436
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									438,431	412,952
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									64,660	58,956
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,408	7,556
Electric
Disaggregation of Revenue [Line Items]
Operating revenues									351,725	335,123
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues									35,574	31,568
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									347,712	331,190
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									4,013	3,933
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									125,369	121,477
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									141,596	136,236
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									37,765	34,353
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,408	7,556
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues									865,222	823,247
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									45,449	43,238
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues									12,726	14,579
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									865,357	817,095
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									(135)	6,152
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									483,452	457,959
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									296,835	276,716
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									26,895	24,603
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Operating revenues									84,192	78,018
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(56,252)	(50,905)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									101,665	89,159
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,164	9,159
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,708	11,543
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Operating revenues									17,687	18,865
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									83,972	77,821
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									220	197
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues									2,189,651	1,925,185
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(1,066)	(669)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(525,749)	(465,969)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									1,088,633	968,755
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,627,833	1,423,068
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									2,189,651	1,925,185
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									0	0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues									1,845,896	1,369,772
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(3,370)	(1,681)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,266,196	926,875
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									531,882	392,544
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues									131	525
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									1,794,839	1,318,263
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									51,057	51,509
Other
Disaggregation of Revenue [Line Items]
Operating revenues									90	207
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(16,461)	(11,052)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues									16,551	11,259
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									90	207
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0	$ 0