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Summary of Significant Accounting Policies - Average Depreciation Rates (Detail) | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Natural gas distribution [Member] | Delmarva [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.60% | 2.50% | 2.50%
Natural gas distribution [Member] | Florida [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 3.50% | 3.50% | 3.20%
Natural gas transmission [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.50% | 2.60% | 2.70%
Electric distribution [Member] | Florida [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 4.20% | 4.20% | 3.80%

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- Definition Composite depreciation rate for public utility plants in service. + References No definition available. + Details
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