Environmental Commitments and Contingencies - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013 sites	Sep. 30, 2012	Dec. 31, 2012
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	6
Environmental liabilities	$ 8,838,000		$ 9,114,000
Amount paid for funding requirements	276,000	345,000
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
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,300,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	9,100,000
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
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs	4,900,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	100,000
Regulatory assets for future recovery of environmental costs	$ 339,000