Environmental Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015 site	Mar. 31, 2014	Dec. 31, 2014
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites	7
Environmental liabilities	$ 9,147,000		$ 8,923,000
Amount paid for funding requirements	49,000	26,000
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	300,000
Regulatory Assets	273,000
Estimated costs of remediation range, minimum	300,000
Estimated costs of remediation range, maximum	500,000
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense	443,000
Additional remediation costs	100,000
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	10,100,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	9,800,000
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	4,200,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	369,000
Regulatory Assets	$ 216,000