Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment by Classification (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, plant and equipment
Total property, plant and equipment	$ 1,007,489	$ 870,125
Less: Accumulated depreciation and amortization	(215,313)	(193,369)
Plus: Construction work in progress	62,774	13,006
Net property, plant and equipment	854,950	689,762
Natural gas distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	207,127	193,071
Natural gas distribution | Florida
Property, plant and equipment
Total property, plant and equipment	286,538	234,344
Natural gas transmission | Delmarva
Property, plant and equipment
Total property, plant and equipment	249,274	243,560
Natural gas transmission | Florida
Property, plant and equipment
Total property, plant and equipment	20,291	18,240
Electric distribution | Florida
Property, plant and equipment
Total property, plant and equipment	79,526	77,640
Propane distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	66,403	61,390
Propane distribution | Florida
Property, plant and equipment
Total property, plant and equipment	24,589	23,142
Propane distribution | OHIO
Property, plant and equipment
Total property, plant and equipment	54,607	0
Other unregulated energy
Property, plant and equipment
Total property, plant and equipment	135	241
Other
Property, plant and equipment
Total property, plant and equipment	$ 18,999	$ 18,497