Summary of Significant Accounting Policies - Average Depreciation Rates (Detail)	12 Months Ended
	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%