Summary of Significant Accounting Policies - Average Depreciation Rates (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Natural gas distribution | Delmarva
Property, Plant and Equipment [Line Items]
Average depreciation rates	2.50%	2.50%	2.40%
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	2.80%	2.70%	2.70%
Natural gas transmission | Florida
Property, Plant and Equipment [Line Items]
Average depreciation rates	3.50%	3.90%	4.00%
Electric distribution | Florida
Property, Plant and Equipment [Line Items]
Average depreciation rates	3.40%	3.50%	3.50%