Property, plant and equipment (Details 3) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, plant and equipment [Line Items]
Interest capitalized		$ 0	$ 0	$ 4,381
AFUDC - borrowed		966	914	4,907
AFUDC - equity		909	565	7,971
Property, plant and equipment		6,770,829	6,510,229
Less accumulated depreciation, depletion and amortization		2,691,641	2,578,902	2,489,322
Net property, plant and equipment		4,079,188	3,931,327	3,898,380
Electric:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	1,982,264	1,888,613	1,786,148
Natural gas distribution:
Property, plant and equipment [Line Items]
Property, plant and equipment	[1]	2,319,845	2,179,413	2,076,581
Pipeline and midstream:
Property, plant and equipment [Line Items]
Property, plant and equipment		700,284	672,199	758,729
Construction materials and contracting:
Property, plant and equipment [Line Items]
Property, plant and equipment		1,560,048	1,549,375	1,553,428
Construction services:
Property, plant and equipment [Line Items]
Property, plant and equipment		177,265	171,361	163,279
Other:
Property, plant and equipment [Line Items]
Property, plant and equipment		31,123	49,268	$ 49,537
Regulated: | Electric: | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,034,765	1,036,373
Weighted average depreciable life in years		39 years
Regulated: | Electric: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 415,543	398,382
Weighted average depreciable life in years		44 years
Regulated: | Electric: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 296,941	284,048
Weighted average depreciable life in years		57 years
Regulated: | Electric: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 117,906	62,212
Regulated: | Electric: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 117,109	107,598
Weighted average depreciable life in years		15 years
Regulated: | Natural gas distribution: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 1,831,795	1,718,633
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		$ 19,823	19,934
Regulated: | Natural gas distribution: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 468,227	440,846
Weighted average depreciable life in years		18 years
Regulated: | Pipeline and midstream: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 516,932	490,143
Weighted average depreciable life in years		53 years
Regulated: | Pipeline and midstream: | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 37,837	37,831
Weighted average depreciable life in years		20 years
Regulated: | Pipeline and midstream: | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 45,629	45,350
Weighted average depreciable life in years		62 years
Regulated: | Pipeline and midstream: | Construction in progress
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 17,488	16,507
Regulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 41,054	40,873
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		$ 31,678	31,682
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		$ 17	13
Nonregulated: | Pipeline and midstream: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 9,649	9,800
Weighted average depreciable life in years		10 years
Nonregulated: | Construction materials and contracting: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 95,745	94,625
Nonregulated: | Construction materials and contracting: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 102,435	102,347
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		$ 947,979	930,471
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		$ 7,750	16,181
Nonregulated: | Construction materials and contracting: | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	[2]	406,139	405,751
Nonregulated: | Construction services: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		5,216	5,346
Nonregulated: | Construction services: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 27,351	26,693
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		$ 137,924	132,217
Weighted average depreciable life in years		6 years
Nonregulated: | Construction services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 6,774	7,105
Weighted average depreciable life in years		4 years
Nonregulated: | Other: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 2,837	2,837
Nonregulated: | Other: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment		$ 28,286	$ 46,431
Weighted average depreciable life in years		19 years

[1] Includes allocations of common utility property

[2] Depleted on the units-of-production method based on recoverable aggregate reserves.