Environmental Commitments and Contingencies - Additional information (Detail) (USD $)	9 Months Ended			9 Months Ended			6 Months Ended			9 Months Ended				9 Months Ended
	Sep. 30, 2012 site	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 West Palm Beach Florida [Member]	Sep. 30, 2012 Sanford Florida [Member]	Sep. 30, 2012 Key West Florida [Member]	Jun. 30, 2012 Pensacola Florida [Member] Maximum [Member]	Jun. 30, 2012 Salisbury Maryland [Member]	Jun. 30, 2012 Winter Haven Florida [Member]	Sep. 30, 2012 Winter Haven Florida [Member] Maximum [Member]	Sep. 30, 2012 Winter Haven Florida [Member] Minimum [Member]	Sep. 30, 2012 FPU [Member]	Sep. 30, 2012 FPU [Member] Manufactured Gas Plant [Member]	Sep. 30, 2012 FPU [Member] Sanford Florida [Member]	Sep. 30, 2012 Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	6
Environmental liabilities	$ 9,170,000		$ 9,492,000									$ 10,700,000			$ 207,000
Approval of recovery of environmental costs												14,000,000
Environmental costs recovered												8,600,000
Regulatory assets for future recovery of environmental costs													5,400,000		702,000
Estimated costs of remediation range, minimum				4,600,000
Estimated costs of remediation range, maximum				15,700,000	13,000,000									650,000
Environmental remediation expense percent														5.00%
Amount paid for funding requirements	345,000	525,000												650,000
Cost of remedy for settlements of claims					20,000,000
Environmental Remediation Expense	443,000				24,000
Costs to resolve liability					650,000	100,000	5,000
Period of regulatory inactivity						17 years
Monitoring well remaining maximum cost								5,000
Remedial action time period										3 years	2 years
Additional remediation costs	100,000
Corrective measures cost									$ 1,000,000