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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.

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- Definition The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 14 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.14) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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