Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Operating revenues	$ 1,209,875	$ 1,280,787	$ 1,064,597	$ 976,293	$ 1,165,239	$ 1,272,548	$ 1,067,639	$ 937,925	$ 4,531,552	$ 4,443,351	$ 4,128,828
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(64,307)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(465,969)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									132,397
Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,159
Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,543
Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									968,755
Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,423,068
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									926,875
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									392,544
Other
Disaggregation of Revenue [Line Items]
Operating revenues									76,796
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									4,469,761
Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									61,791
Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									579,436
Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									412,952
Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									58,956
Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,556
Electric
Disaggregation of Revenue [Line Items]
Operating revenues									335,123
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0
Electric | Other
Disaggregation of Revenue [Line Items]
Operating revenues									31,568
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									331,190
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									3,933
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									121,477
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									136,236
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									34,353
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									7,556
Natural gas distribution
Disaggregation of Revenue [Line Items]
Operating revenues									823,247
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									43,238
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Operating revenues									14,579
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									817,095
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									6,152
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									457,959
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									276,716
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									24,603
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Operating revenues									0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Operating revenues									78,018
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(50,905)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Operating revenues									89,159
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									9,159
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Operating revenues									11,543
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Operating revenues									18,865
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									77,821
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									197
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Operating revenues									1,925,185
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(669)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(465,969)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Operating revenues									968,755
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Operating revenues									1,423,068
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Operating revenues									0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									1,925,185
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									0
Construction services
Disaggregation of Revenue [Line Items]
Operating revenues									1,369,772
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(1,681)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									926,875
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Operating revenues									392,544
Construction services | Other
Disaggregation of Revenue [Line Items]
Operating revenues									525
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									1,318,263
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									51,509
Other
Disaggregation of Revenue [Line Items]
Operating revenues									207
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Operating revenues									(11,052)
Other | Other
Disaggregation of Revenue [Line Items]
Operating revenues									11,259
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Operating revenues									207
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0