Environmental Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 site	Dec. 31, 2013	Dec. 31, 2012
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	6
Environmental liabilities	$ 8,923,000	$ 9,155,000
Amount paid for funding requirements	233,000	(41,000)	607,000
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	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]
Estimated costs of remediation range, minimum	300,000
Estimated costs of remediation range, maximum	500,000
FPU
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,100,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	9,700,000
FPU | Manufactured Gas Plant
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs	4,300,000
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	382,000
Regulatory assets for future recovery of environmental costs	$ 313,000