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Summary of Significant Accounting Policies (Details 3) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Allowance for Funds Used During Construction, Capitalized Cost of Equity and Interest | $ 15,100,000 |  | $ 17,600,000 |  | $ 17,400,000 |
Property, plant and equipment | 7,646,222,000 |  | 7,218,503,000 |  |  |
Less accumulated depreciation, depletion and amortization | 3,361,208,000 |  | 3,103,323,000 |  | 2,872,465,000 |
Net property, plant and equipment | 4,285,014,000 |  | 4,115,180,000 |  | 3,894,117,000 |
Mcf equivalent average rate for amortization under units-of-production method (in dollars per Mcf) | 2.04 |  | 1.77 |  | 1.64 |
Capitalized Costs of Unproved Properties Excluded from Amortization, Cumulative | 232,462,000 |  | 182,402,000 |  | 178,214,000 |
Electric [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,068,524,000 | [1] | 1,027,034,000 | [1] | 941,791,000 | [1]
Natural gas distribution [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,568,866,000 | [1] | 1,508,845,000 | [1] | 1,456,208,000 | [1]
Pipeline and energy services [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 719,291,000 |  | 683,807,000 |  | 675,199,000 |
Exploration and production [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 2,615,146,000 |  | 2,356,938,000 |  | 2,028,794,000 |
Construction materials and contracting [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,499,852,000 |  | 1,486,375,000 |  | 1,514,989,000 |
Construction services [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 124,796,000 |  | 122,940,000 |  | 116,236,000 |
All Other Segments [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 49,747,000 |  | 32,564,000 |  | 33,365,000 |
Regulated Operation [Member] | Electric [Member] | Electric Generation Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 546,783,000 |  | 538,071,000 |  |  |
Weighted Average Depreciable Life in Years | 47 |  |  |  |  |
Regulated Operation [Member] | Electric [Member] | Electric Distribution [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 255,232,000 |  | 243,205,000 |  |  |
Weighted Average Depreciable Life in Years | 36 |  |  |  |  |
Regulated Operation [Member] | Electric [Member] | Electric Transmission [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 179,580,000 |  | 161,972,000 |  |  |
Weighted Average Depreciable Life in Years | 44 |  |  |  |  |
Regulated Operation [Member] | Electric [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 86,929,000 |  | 83,786,000 |  |  |
Weighted Average Depreciable Life in Years | 13 |  |  |  |  |
Regulated Operation [Member] | Natural gas distribution [Member] | Distribution [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,257,360,000 |  | 1,223,239,000 |  |  |
Weighted Average Depreciable Life in Years | 38 |  |  |  |  |
Regulated Operation [Member] | Natural gas distribution [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 311,506,000 |  | 285,606,000 |  |  |
Weighted Average Depreciable Life in Years | 23 |  |  |  |  |
Regulated Operation [Member] | Pipeline and energy services [Member] | Transmission [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 386,227,000 |  | 357,395,000 |  |  |
Weighted Average Depreciable Life in Years | 52 |  |  |  |  |
Regulated Operation [Member] | Pipeline and energy services [Member] | Gathering [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 42,378,000 |  | 41,931,000 |  |  |
Weighted Average Depreciable Life in Years | 19 |  |  |  |  |
Regulated Operation [Member] | Pipeline and energy services [Member] | Storage [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 41,908,000 |  | 33,967,000 |  |  |
Weighted Average Depreciable Life in Years | 51 |  |  |  |  |
Regulated Operation [Member] | Pipeline and energy services [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 36,179,000 |  | 33,938,000 |  |  |
Weighted Average Depreciable Life in Years | 29 |  |  |  |  |
Unregulated Operation [Member] | Pipeline and energy services [Member] | Gathering [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 198,864,000 |  | 203,064,000 |  |  |
Weighted Average Depreciable Life in Years | 17 |  |  |  |  |
Unregulated Operation [Member] | Pipeline and energy services [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 13,735,000 |  | 13,512,000 |  |  |
Weighted Average Depreciable Life in Years | 10 |  |  |  |  |
Unregulated Operation [Member] | Exploration and production [Member] | Oil and natural gas properties [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 2,577,576,000 | [2] | 2,320,967,000 | [2] |  |
Unregulated Operation [Member] | Exploration and production [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 37,570,000 |  | 35,971,000 |  |  |
Weighted Average Depreciable Life in Years | 9 |  |  |  |  |
Unregulated Operation [Member] | Construction materials and contracting [Member] | Land [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 126,790,000 |  | 124,018,000 |  |  |
Unregulated Operation [Member] | Construction materials and contracting [Member] | Building and Building Improvements [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 67,627,000 |  | 65,003,000 |  |  |
Weighted Average Depreciable Life in Years | 20 |  |  |  |  |
Unregulated Operation [Member] | Construction materials and contracting [Member] | Machinery, vehicles and equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 902,136,000 |  | 899,365,000 |  |  |
Weighted Average Depreciable Life in Years | 12 |  |  |  |  |
Unregulated Operation [Member] | Construction materials and contracting [Member] | Construction in Progress [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 8,085,000 |  | 4,879,000 |  |  |
Unregulated Operation [Member] | Construction materials and contracting [Member] | Aggregate reserves [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 395,214,000 | [3] | 393,110,000 | [3] |  |
Unregulated Operation [Member] | Construction services [Member] | Land [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 4,706,000 |  | 4,526,000 |  |  |
Unregulated Operation [Member] | Construction services [Member] | Building and Building Improvements [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 15,001,000 |  | 14,101,000 |  |  |
Weighted Average Depreciable Life in Years | 22 |  |  |  |  |
Unregulated Operation [Member] | Construction services [Member] | Machinery, vehicles and equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 95,891,000 |  | 94,252,000 |  |  |
Weighted Average Depreciable Life in Years | 7 |  |  |  |  |
Unregulated Operation [Member] | Construction services [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 9,198,000 |  | 10,061,000 |  |  |
Weighted Average Depreciable Life in Years | 4 |  |  |  |  |
Unregulated Operation [Member] | All Other Segments [Member] | Land [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 2,837,000 |  | 2,837,000 |  |  |
Unregulated Operation [Member] | All Other Segments [Member] | Other Capitalized Property Plant and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | $ 46,910,000 |  | $ 29,727,000 |  |  |
Weighted Average Depreciable Life in Years | 24 |  |  |  |  |
[1] | Includes allocations of common utility property.
[2] | Amortized on the units-of-production method based on total proved reserves at an Mcf equivalent average rate of $2.04, $1.77 and $1.64 for the years ended December 31, 2011, 2010 and 2009, respectively. Includes natural gas and oil properties accounted for under the full-cost method, of which $232.5 and $182.4 million were excluded from amortization at December 31, 2011 and 2010, respectively.
[3] | Depleted on the units-of-production method.

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