Environmental Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 site	Dec. 31, 2011	Dec. 31, 2010
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	6
Environmental liabilities	$ 9,114,000	$ 9,492,000
Amount paid for funding requirements	607,000	645,000	290,000
West Palm Beach Florida [Member]
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 [Member]
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
Environmental Remediation Expense	24,000
Costs to resolve liability	650,000
Key West Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	50,000
Period of regulatory inactivity	17 years
Key West Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	8,000
Pensacola Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Costs to resolve liability	5,000
Salisbury Maryland [Member]
Environmental Commitments And Contingencies [Line Items]
Monitoring well remaining maximum cost	5,000
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental Remediation Expense	443,000
Additional remediation costs	100,000
Corrective measures cost	1,000,000
Winter Haven Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Remedial action time period	3 years
Winter Haven Florida [Member] | Minimum [Member]
Environmental Commitments And Contingencies [Line Items]
Remedial action time period	2 years
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,500,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	8,700,000
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs	5,300,000
FPU [Member] | Sanford Florida [Member]
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 [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	170,000
Regulatory assets for future recovery of environmental costs	$ 612,000