Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($) site claim	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site	7
Environmental liabilities	$ 8,843,000		$ 8,942,000
Amount paid for funding requirements	99,000	$ 49,000
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	273,000
Accrual for Environmental Loss Contingencies	186,000
Regulatory Assets	269,000
Estimated costs of remediation range, minimum	273,000
Estimated costs of remediation range, maximum	465,000
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense	443,000
Additional remediation costs	100,000
Salisbury Maryland [Member]
Environmental Commitments And Contingencies [Line Items]
Maximum Costs Of Monitoring Well	5,000
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
Loss Contingency, Pending Claims, Number | claim	2
Environmental remediation expense	$ 24,000
Key West Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Period Of Regulatory Inactivity	17 years
Environmental Exit Costs, Anticipated Cost	$ 50,000
Aspire Energy [Member]
Environmental Commitments And Contingencies [Line Items]
Number Of Environmental Sites | site	8
Cost of remedy for settlements of claims	$ 1,400,000
Minimum Environmental Remediation Expenditures	431,250
Business Combination, Indemnification Assets, Range of Outcomes, Value, High	1,700,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,000,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	10,200,000
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%
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets	$ 3,800,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	353,000
Regulatory Assets	58,000
Maximum [Member] | Key West Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost	8,000
Maximum [Member] | Pensacola Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Anticipated Cost	$ 5,000