Revenue from contracts with customers Disaggregation of revenue (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues		$ 976,293	$ 937,925
Intersegment operating revenues		0	$ 0
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	(34,270)
Other
Disaggregation of Revenue [Line Items]
Revenues		18,187
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(24,509)
Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		964,542
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		11,751
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		32,997
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		2,270
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		3,134
Contracting services
Disaggregation of Revenue [Line Items]
Revenues		74,064
Construction materials
Disaggregation of Revenue [Line Items]
Revenues		173,591
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		233,821
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		87,181
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		228,069
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		151,592
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		16,579
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		1,836
Electric
Disaggregation of Revenue [Line Items]
Revenues		87,404
Electric | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues		8,252
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		0
Electric | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		88,742
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		(1,338)
Electric | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		35,183
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		34,701
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		8,770
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		1,836
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues		332,664
Natural gas distribution | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues		3,999
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		0
Natural gas distribution | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		332,764
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		(100)
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		11,179
Natural gas distribution | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		192,886
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		116,891
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		7,809
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Revenues		8,833
Pipeline and midstream | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Revenues		3,326
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(21,759)
Pipeline and midstream | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		8,789
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		44
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		21,818
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		2,270
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		3,134
Pipeline and midstream | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Pipeline and midstream | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues		213,284
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	(34,270)
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(101)
Construction materials and contracting | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		213,284
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		74,064
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		173,591
Construction materials and contracting | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction materials and contracting | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services
Disaggregation of Revenue [Line Items]
Revenues		334,050
Construction services | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues		(86)
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(11)
Construction services | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		320,905
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		13,145
Construction services | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		233,821
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		87,181
Construction services | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Construction services | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other
Disaggregation of Revenue [Line Items]
Revenues		58
Other | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	[1]	0
Other | Other
Disaggregation of Revenue [Line Items]
Revenues		2,696
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Intersegment operating revenues		(2,638)
Other | Revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues		58
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Contracting services
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Construction materials
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues		0
Other | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues		$ 0

[1] Intrasegment revenues are presented within the construction materials and contracting segment to highlight the focus on vertical integration as this segment sells materials to both third parties and internal customers. Due to consolidation requirements, these revenues must be eliminated against construction materials to arrive at the external operating revenue total for the segment.