Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2016 USD ($) site claim	Sep. 30, 2016 USD ($) site claim	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	$ 8,682,000	$ 8,682,000	$ 8,942,000
Restructuring Reserve, Accrual Adjustment		(431,250)
Proceeds from Legal Settlements		500,000
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	273,000	273,000
Regulatory Assets	267,000	267,000
Accrual for Environmental Loss Contingencies	$ 156,000	156,000
Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
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
Number of Wells		2
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]
Number of Wells		1
Maximum Costs Of Monitoring Well		$ 5,000
Aspire Energy [Member]
Environmental Commitments And Contingencies [Line Items]
Cost of remedy for settlements of claims		$ 1,600,000
Number Of Environmental Sites | site	8	8
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		$ 10,500,000
FPU [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense percent		5.00%
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets	3,500,000	$ 3,500,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	298,000	298,000
Minimum [Member] | Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		273,000
Minimum [Member] | West Palm Beach Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		4,500,000
Maximum [Member] | Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	$ 465,000
Maximum [Member] | West Palm Beach Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		15,400,000
Maximum [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		13,000,000
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
Maximum [Member] | FPU [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		$ 650,000