Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment by Classification (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment
Total property, plant and equipment	$ 1,312,117	$ 1,175,595
Less: Accumulated depreciation and amortization	(270,599)	(245,207)
Plus: Construction work in progress	84,509	56,276
Net property, plant and equipment	1,126,027	986,664
OHIO
Property, plant and equipment
Total property, plant and equipment	66,037	61,383
Natural gas distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	234,654	220,083
Natural gas distribution | Florida
Property, plant and equipment
Total property, plant and equipment	354,495	331,281
Natural gas transmission | Delmarva
Property, plant and equipment
Total property, plant and equipment	357,264	285,746
Natural gas transmission | Florida
Property, plant and equipment
Total property, plant and equipment	27,096	27,018
Electric distribution | Florida
Property, plant and equipment
Total property, plant and equipment	100,227	93,553
Propane distribution | Delmarva
Property, plant and equipment
Total property, plant and equipment	79,139	73,686
Propane distribution | Florida
Property, plant and equipment
Total property, plant and equipment	29,038	26,359
CHP | Florida
Property, plant and equipment
Total property, plant and equipment	35,239	35,237
Other unregulated energy
Property, plant and equipment
Total property, plant and equipment	1,229	135
Other
Property, plant and equipment
Total property, plant and equipment	$ 27,699	$ 21,114