Property, plant and equipment (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, plant and equipment [Line Items]
AFUDC - borrowed	$ 2,807	$ 2,290	$ 966
AFUDC - equity	698	1,897	909
Property, plant and equipment	7,908,628	7,397,321
Less accumulated depreciation, depletion and amortization	2,991,486	2,818,644	2,691,641
Net property, plant and equipment	4,917,142	4,578,677	4,079,188
Electric
Property, plant and equipment [Line Items]
Property, plant and equipment	2,227,145	2,148,569	1,982,264
Natural gas distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	2,688,123	2,499,093	2,319,845
Pipeline and midstream
Property, plant and equipment [Line Items]
Property, plant and equipment	834,215	764,959	700,284
Construction materials and contracting
Property, plant and equipment [Line Items]
Property, plant and equipment	1,910,562	1,768,006	1,560,048
Construction services
Property, plant and equipment [Line Items]
Property, plant and equipment	213,370	188,586	177,265
Other
Property, plant and equipment [Line Items]
Property, plant and equipment	35,213	28,108	$ 31,123
Regulated: | Electric | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 1,139,059	1,131,484
Weighted average depreciable life in years	48 years
Regulated: | Electric | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 443,780	430,750
Weighted average depreciable life in years	46 years
Regulated: | Electric | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 445,485	302,315
Weighted average depreciable life in years	65 years
Regulated: | Electric | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 66,664	161,893
Regulated: | Electric | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 132,157	122,127
Weighted average depreciable life in years	15 years
Regulated: | Natural gas distribution | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 2,133,249	1,981,356
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	$ 39,506	21,028
Regulated: | Natural gas distribution | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 515,368	496,708
Weighted average depreciable life in years	17 years
Regulated: | Pipeline and midstream | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 636,796	585,594
Weighted average depreciable life in years	46 years
Regulated: | Pipeline and midstream | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 35,661	37,829
Weighted average depreciable life in years	20 years
Regulated: | Pipeline and midstream | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 50,001	49,101
Weighted average depreciable life in years	53 years
Regulated: | Pipeline and midstream | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 22,597	5,915
Regulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 48,340	45,763
Weighted average depreciable life in years	16 years
Nonregulated: | Pipeline and midstream | Gathering and processing
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 31,148	31,094
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	$ 154	86
Nonregulated: | Pipeline and midstream | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 9,518	9,577
Weighted average depreciable life in years	10 years
Nonregulated: | Construction materials and contracting | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 127,729	109,541
Nonregulated: | Construction materials and contracting | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 122,064	114,905
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	$ 1,180,343	1,090,790
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	$ 25,018	22,507
Nonregulated: | Construction materials and contracting | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	455,408	430,263
Nonregulated: | Construction services | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	7,146	5,216
Nonregulated: | Construction services | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 31,735	29,795
Weighted average depreciable life in years	24 years
Nonregulated: | Construction services | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 156,537	145,859
Weighted average depreciable life in years	6 years
Nonregulated: | Construction services | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 17,952	7,716
Weighted average depreciable life in years	2 years
Nonregulated: | Other | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 2,648	2,648
Nonregulated: | Other | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	$ 32,565	$ 25,461
Weighted average depreciable life in years	14 years