Revenue from contracts with customers Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 1,280,787	$ 1,272,548	$ 3,321,678	$ 3,278,112
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(7,171)	$ (5,586)	(41,661)	$ (37,581)
Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(204,040)		(363,877)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	32,241		96,609
Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,320		6,900
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,795		8,563
Contracting services
Disaggregation of Revenue [Line Items]
Revenues	409,006		730,628
Construction materials
Disaggregation of Revenue [Line Items]
Revenues	538,962		1,100,185
Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	217,474		667,664
Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	100,988		283,432
Other
Disaggregation of Revenue [Line Items]
Revenues	18,166		55,578
Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	1,265,638		3,281,257
Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	15,149		40,421
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	75,249		398,758
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	64,433		289,521
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	13,159		43,191
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	2,056		5,766
Electric
Disaggregation of Revenue [Line Items]
Revenues	86,080		251,984
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0		0
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	6,158		22,836
Electric | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	84,635		251,331
Electric | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,445		653
Electric | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	31,424		93,359
Electric | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	36,259		103,636
Electric | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	8,738		25,734
Electric | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	2,056		5,766
Natural gas distribution
Disaggregation of Revenue [Line Items]
Revenues	92,248		554,452
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	0		0
Natural gas distribution | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	10,841		32,104
Natural gas distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	3,208		10,821
Natural gas distribution | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	90,469		551,666
Natural gas distribution | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	1,779		2,786
Natural gas distribution | Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues	43,825		305,399
Natural gas distribution | Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues	28,174		185,885
Natural gas distribution | Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues	4,421		17,457
Natural gas distribution | Other utility sales
Disaggregation of Revenue [Line Items]
Revenues	0		0
Pipeline and midstream
Disaggregation of Revenue [Line Items]
Revenues	29,071		61,965
Pipeline and midstream | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,187)		(31,485)
Pipeline and midstream | Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues	21,400		64,505
Pipeline and midstream | Natural gas gathering
Disaggregation of Revenue [Line Items]
Revenues	2,320		6,900
Pipeline and midstream | Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues	2,795		8,563
Pipeline and midstream | Other
Disaggregation of Revenue [Line Items]
Revenues	5,701		13,353
Pipeline and midstream | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	29,029		61,836
Pipeline and midstream | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	42		129
Construction materials and contracting
Disaggregation of Revenue [Line Items]
Revenues	743,763		1,466,435
Construction materials and contracting | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(165)		(501)
Construction materials and contracting | Intrasegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(204,040)		(363,877)
Construction materials and contracting | Contracting services
Disaggregation of Revenue [Line Items]
Revenues	409,006		730,628
Construction materials and contracting | Construction materials
Disaggregation of Revenue [Line Items]
Revenues	538,962		1,100,185
Construction materials and contracting | Other
Disaggregation of Revenue [Line Items]
Revenues	0		0
Construction materials and contracting | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	743,763		1,466,435
Construction materials and contracting | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	0		0
Construction services
Disaggregation of Revenue [Line Items]
Revenues	329,578		986,649
Construction services | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(782)		(1,332)
Construction services | Inside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	217,474		667,664
Construction services | Outside specialty contracting
Disaggregation of Revenue [Line Items]
Revenues	100,988		283,432
Construction services | Other
Disaggregation of Revenue [Line Items]
Revenues	15		32
Construction services | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	317,695		949,796
Construction services | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	11,883		36,853
Other
Disaggregation of Revenue [Line Items]
Revenues	47		193
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues	(3,037)		(8,343)
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	3,084		8,536
Other | Revenues from contracts with customers
Disaggregation of Revenue [Line Items]
Revenues	47		193
Other | Revenues out of scope
Disaggregation of Revenue [Line Items]
Revenues	$ 0		$ 0