Property, plant and equipment (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment [Line Items]
AFUDC - borrowed	$ 2,290	$ 966	$ 914
AFUDC - equity	1,897	909	565
Property, plant and equipment	7,397,321	6,770,829
Less accumulated depreciation, depletion and amortization	2,818,644	2,691,641	2,578,902
Net property, plant and equipment	4,578,677	4,079,188	3,931,327
Electric
Property, plant and equipment [Line Items]
Property, plant and equipment	2,148,569	1,982,264	1,888,613
Natural gas distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	2,499,093	2,319,845	2,179,413
Pipeline and midstream
Property, plant and equipment [Line Items]
Property, plant and equipment	764,959	700,284	672,199
Construction materials and contracting
Property, plant and equipment [Line Items]
Property, plant and equipment	1,768,006	1,560,048	1,549,375
Construction services
Property, plant and equipment [Line Items]
Property, plant and equipment	188,586	177,265	171,361
Other
Property, plant and equipment [Line Items]
Property, plant and equipment	28,108	31,123	$ 49,268
Regulated: | Electric | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,131,484	1,034,765
Weighted average depreciable life in years	49 years
Regulated: | Electric | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 430,750	415,543
Weighted average depreciable life in years	46 years
Regulated: | Electric | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 302,315	296,941
Weighted average depreciable life in years	64 years
Regulated: | Electric | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 161,893	117,906
Regulated: | Electric | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 122,127	117,109
Weighted average depreciable life in years	13 years
Regulated: | Natural gas distribution | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,981,356	1,831,795
Weighted average depreciable life in years	47 years
Regulated: | Natural gas distribution | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 21,028	19,823
Regulated: | Natural gas distribution | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 496,708	468,227
Weighted average depreciable life in years	16 years
Regulated: | Pipeline and midstream | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 585,594	516,932
Weighted average depreciable life in years	54 years
Regulated: | Pipeline and midstream | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 37,829	37,837
Weighted average depreciable life in years	20 years
Regulated: | Pipeline and midstream | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 49,101	45,629
Weighted average depreciable life in years	61 years
Regulated: | Pipeline and midstream | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 5,915	17,488
Regulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 45,763	41,054
Weighted average depreciable life in years	33 years
Nonregulated: | Pipeline and midstream | Gathering and processing
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 31,094	31,678
Weighted average depreciable life in years	19 years
Nonregulated: | Pipeline and midstream | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 86	17
Nonregulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 9,577	9,649
Weighted average depreciable life in years	10 years
Nonregulated: | Construction materials and contracting | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 109,541	95,745
Nonregulated: | Construction materials and contracting | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 114,905	102,435
Weighted average depreciable life in years	20 years
Nonregulated: | Construction materials and contracting | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,090,790	947,979
Weighted average depreciable life in years	12 years
Nonregulated: | Construction materials and contracting | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 22,507	7,750
Nonregulated: | Construction materials and contracting | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	430,263	406,139
Nonregulated: | Construction services | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	5,216	5,216
Nonregulated: | Construction services | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 29,795	27,351
Weighted average depreciable life in years	25 years
Nonregulated: | Construction services | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 145,859	137,924
Weighted average depreciable life in years	6 years
Nonregulated: | Construction services | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 7,716	6,774
Weighted average depreciable life in years	3 years
Nonregulated: | Other | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 2,648	2,837
Nonregulated: | Other | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 25,461	$ 28,286
Weighted average depreciable life in years	14 years