Environmental Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) site claim	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2015 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site	7
Environmental liabilities	$ 8,942,000	$ 8,923,000
Regulatory Assets	85,787,000	87,103,000
Amount paid for funding requirements	174,000	$ 233,000	$ (41,000)
West Palm Beach Florida
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	4,500,000
Estimated costs of remediation range, maximum	15,400,000
Sanford Florida
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, maximum	13,000,000
Cost of remedy for settlements of claims	$ 20,000,000
Loss Contingency, Pending Claims, Number | claim	2
Environmental Remediation Expense	$ 24,000
Costs to resolve liability	650,000
Key West Florida
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	$ 50,000
Period of regulatory inactivity	17 years
Key West Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	$ 8,000
Pensacola Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	5,000
Salisbury Maryland
Environmental Commitments And Contingencies [Line Items]
Monitoring well remaining maximum cost	5,000
Winter Haven Florida
Environmental Commitments And Contingencies [Line Items]
Environmental Remediation Expense	443,000
Additional remediation costs	100,000
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	223,000			$ 273,000
Regulatory Assets	273,000
Estimated costs of remediation range, minimum	273,000
Estimated costs of remediation range, maximum	$ 465,000
Aspire Energy [Member]
Environmental Commitments And Contingencies [Line Items]
Number Of Environmental Sites | site	8
Cost of remedy for settlements of claims	$ 1,000,000
Minimum Environmental Remediation Expenditures	431,250
Business Combination, Indemnification Assets, Range of Outcomes, Value, High	1,650,000
FPU
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,000,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	10,100,000
FPU | Manufactured Gas Plant
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs	3,900,000
FPU | Sanford Florida
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, maximum	$ 650,000
Environmental remediation expense percent	5.00%
Amount paid for funding requirements	$ 650,000
Chesapeake
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	375,000
Regulatory assets for future recovery of environmental costs	$ 87,000