Environmental Commitments and Contingencies - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014 site	Sep. 30, 2013	Dec. 31, 2013
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	6
Environmental liabilities	$ 9,022,000		$ 9,155,000
Amount paid for funding requirements	134,000	276,000
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	10,200,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	9,500,000
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	4,500,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	434,000
Regulatory assets for future recovery of environmental costs	$ 408,000