Property, plant and equipment (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Property, plant and equipment [Line Items]
Interest capitalized	$ 8,659		$ 10,821		$ 9,753
AFUDC - borrowed	2,483		1,666		2,950
AFUDC - equity	4,530		2,587		4,896
Property, plant and equipment	8,107,751		7,646,222
Less accumulated depreciation, depletion and amortization	3,608,912		3,361,208		3,103,323
Net property, plant and equipment	4,498,839		4,285,014		4,115,180
BOE average rate for amortization under units-of-production method (in dollars per BOE)	15.28		12.25		10.64
Oil and natural gas properties not subject to amortization	191,794		232,462		182,402
Electric:
Property, plant and equipment [Line Items]
Property, plant and equipment	1,150,584	[1]	1,068,524	[1]	1,027,034	[1]
Natural gas distribution:
Property, plant and equipment [Line Items]
Property, plant and equipment	1,689,950	[1]	1,568,866	[1]	1,508,845	[1]
Pipeline and energy services:
Property, plant and equipment [Line Items]
Property, plant and equipment	816,533		719,291		683,807
Exploration and production:
Property, plant and equipment [Line Items]
Property, plant and equipment	2,764,560		2,615,146		2,356,938
Construction materials and contracting:
Property, plant and equipment [Line Items]
Property, plant and equipment	1,504,981		1,499,852		1,486,375
Construction services:
Property, plant and equipment [Line Items]
Property, plant and equipment	130,624		124,796		122,940
Other:
Property, plant and equipment [Line Items]
Property, plant and equipment	50,519		49,747		32,564
Regulated: | Electric: | Generation
Property, plant and equipment [Line Items]
Property, plant and equipment	580,567		546,783
Weighted average depreciable life in years	47 years
Regulated: | Electric: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	282,424		255,232
Weighted average depreciable life in years	36 years
Regulated: | Electric: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	190,311		179,580
Weighted average depreciable life in years	44 years
Regulated: | Electric: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	97,282		86,929
Weighted average depreciable life in years	14 years
Regulated: | Natural gas distribution: | Distribution
Property, plant and equipment [Line Items]
Property, plant and equipment	1,329,692		1,257,360
Weighted average depreciable life in years	40 years
Regulated: | Natural gas distribution: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	360,258		311,506
Weighted average depreciable life in years	24 years
Regulated: | Pipeline and energy services: | Transmission
Property, plant and equipment [Line Items]
Property, plant and equipment	416,186		386,227
Weighted average depreciable life in years	52 years
Regulated: | Pipeline and energy services: | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment	42,424		42,378
Weighted average depreciable life in years	19 years
Regulated: | Pipeline and energy services: | Storage
Property, plant and equipment [Line Items]
Property, plant and equipment	42,554		41,908
Weighted average depreciable life in years	51 years
Regulated: | Pipeline and energy services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	38,493		36,179
Weighted average depreciable life in years	29 years
Nonregulated: | Pipeline and energy services: | Gathering
Property, plant and equipment [Line Items]
Property, plant and equipment	259,724		198,864
Weighted average depreciable life in years	16 years
Nonregulated: | Pipeline and energy services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	17,152		13,735
Weighted average depreciable life in years	10 years
Nonregulated: | Exploration and production: | Oil and natural gas properties
Property, plant and equipment [Line Items]
Property, plant and equipment	2,723,356	[2]	2,577,576	[2]
Nonregulated: | Exploration and production: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	41,204		37,570
Weighted average depreciable life in years	8 years
Nonregulated: | Construction materials and contracting: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	126,788		126,790
Nonregulated: | Construction materials and contracting: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	73,884		67,627
Weighted average depreciable life in years	19 years
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	899,592		902,136
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	11,165		8,085
Nonregulated: | Construction materials and contracting: | Aggregate reserves
Property, plant and equipment [Line Items]
Property, plant and equipment	393,552	[3]	395,214	[3]
Nonregulated: | Construction services: | Land
Property, plant and equipment [Line Items]
Property, plant and equipment	4,723		4,706
Nonregulated: | Construction services: | Buildings and improvements
Property, plant and equipment [Line Items]
Property, plant and equipment	16,563		15,001
Weighted average depreciable life in years	23 years
Nonregulated: | Construction services: | Machinery, vehicles and equipment
Property, plant and equipment [Line Items]
Property, plant and equipment	100,445		95,891
Weighted average depreciable life in years	7 years
Nonregulated: | Construction services: | Other
Property, plant and equipment [Line Items]
Property, plant and equipment	8,893		9,198
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	$ 47,682		$ 46,910
Weighted average depreciable life in years	24 years

[1] Includes allocations of common utility property.

[2] Amortized on the units-of-production method based on total proved reserves at a BOE average rate of $15.28, $12.25 and $10.64 for the years ended December 31, 2012, 2011 and 2010, respectively. Includes oil and natural gas properties accounted for under the full-cost method, of which $191.8 million and $232.5 million were excluded from amortization at December 31, 2012 and 2011, respectively.

[3] Depleted on the units-of-production method.