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Summary of Significant Accounting Policies - Average Depreciation Rates (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Natural gas distribution | Delmarva |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.50% | 2.50% | 2.50%
Natural gas distribution | Florida |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.90% | 2.90% | 2.90%
Natural gas transmission | Delmarva |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.70% | 2.80% | 2.70%
Natural gas transmission | Florida |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 2.30% | 3.50% | 3.90%
Electric distribution | Florida |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Average depreciation rates | 3.40% | 3.40% | 3.50%

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