Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment by Classification (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Property, plant and equipment
Total property, plant and equipment	$ 675,714,000	$ 616,105,000
Less: Accumulated depreciation and amortization	(155,378,000)	(137,784,000)
Plus: Construction work in progress	21,445,000	9,383,000
Net property, plant and equipment	541,781,000	487,704,000
Natural gas distribution [Member] | Delmarva [Member]
Property, plant and equipment
Total property, plant and equipment	149,558,000	140,800,000
Natural gas distribution [Member] | Florida [Member]
Property, plant and equipment
Total property, plant and equipment	170,943,000	158,341,000
Natural gas transmission [Member]
Property, plant and equipment
Total property, plant and equipment	202,968,000	173,810,000
Electric distribution [Member] | Florida [Member]
Property, plant and equipment
Total property, plant and equipment	61,960,000	55,839,000
Propane distribution [Member] | Delmarva [Member]
Property, plant and equipment
Total property, plant and equipment	53,156,000	51,250,000
Propane distribution [Member] | Florida [Member]
Property, plant and equipment
Total property, plant and equipment	16,823,000	15,839,000
Other unregulated energy [Member]
Property, plant and equipment
Total property, plant and equipment	239,000	238,000
Other [Member]
Property, plant and equipment
Total property, plant and equipment	$ 20,067,000	$ 19,988,000