Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2016 USD ($) site claim	Jun. 30, 2016 USD ($) site claim	Jun. 30, 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	$ 8,765,000	$ 8,765,000		$ 8,942,000
Amount paid for funding requirements		177,000	$ 73,000
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	273,000	273,000
Accrual for Environmental Loss Contingencies	177,000	177,000
Regulatory Assets	268,000	268,000
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	425,000
Additional remediation costs	100,000	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	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	8
Cost of remedy for settlements of claims	$ 1,600,000
Minimum Environmental Remediation Expenditures	431,250
Business Combination, Indemnification Assets, Range of Outcomes, Value, High	1,700,000	$ 1,700,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	9,900,000	9,900,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	$ 10,300,000
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	$ 3,700,000	3,700,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	314,000	314,000
Regulatory Assets	29,000	$ 29,000
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