Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) claim	Jun. 30, 2017 USD ($) site claim	Dec. 31, 2016 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site		7
Environmental liabilities	$ 8,457,000	$ 8,457,000	$ 8,592,000
West Palm Beach Florida [Member] | Minimum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		4,500,000
West Palm Beach Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		15,400,000
Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		13,000,000
Number of Year reviews to ensure remedy	5 years
Cost of remedy for settlements of claims		$ 20,000,000
Loss Contingency, Pending Claims, Number | claim	2	2
Environmental remediation expense		$ 24,000
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Additional remediation costs	$ 100,000	100,000
Winter Haven Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense		425,000
Seaford [Member] | Minimum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		273,000
Seaford [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		465,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	9,800,000	9,800,000
Approval of recovery of environmental costs		14,000,000
Environmental costs recovered		10,800,000
FPU [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum		$ 650,000
Environmental remediation expense percent		5.00%
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets for future recovery of environmental costs	$ 3,200,000	$ 3,200,000