Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment by Classification (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, plant and equipment
Total property, plant and equipment	$ 1,175,595	$ 1,007,489
Less: Accumulated depreciation and amortization	(245,207)	(215,313)
Plus: Construction work in progress	56,276	62,774
Net property, plant and equipment	986,664	854,950
OHIO
Property, plant and equipment
Total property, plant and equipment	61,383	54,607
Natural gas distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	220,083	207,127
Natural gas distribution | Florida
Property, plant and equipment
Total property, plant and equipment	331,281	286,538
Natural gas transmission | Delmarva
Property, plant and equipment
Total property, plant and equipment	285,746	249,274
Natural gas transmission | Florida
Property, plant and equipment
Total property, plant and equipment	27,018	20,291
Electric distribution | Florida
Property, plant and equipment
Total property, plant and equipment	93,553	79,526
Propane distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	73,686	66,403
Propane distribution | Florida
Property, plant and equipment
Total property, plant and equipment	26,359	24,589
CHP | Florida
Property, plant and equipment
Total property, plant and equipment	35,237	0
Other unregulated energy
Property, plant and equipment
Total property, plant and equipment	135	135
Other
Property, plant and equipment
Total property, plant and equipment	$ 21,114	$ 18,999