Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 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	$ 8,535,000	$ 8,592,000
West Palm Beach Florida [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]
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
Environmental remediation expense	$ 24,000
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense	425,000
Additional remediation costs	100,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	9,800,000
Approval of recovery of environmental costs	14,000,000
Environmental costs recovered	$ 10,700,000
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 for future recovery of environmental costs	$ 3,300,000
Minimum [Member] | West Palm Beach Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	4,500,000
Minimum [Member] | Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	273,000
Maximum [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	13,000,000
Maximum [Member] | Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	465,000
Maximum [Member] | FPU [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	$ 650,000