Environmental Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) site	Dec. 31, 2019 USD ($)
Environmental Commitments And Contingencies [Line Items]
Companys Exposure In Number Of Former Manufactured Gas Plant Sites | site	7
Environmental liabilities	$ 4,299,000	$ 6,468,000
West Palm Beach Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss	14,200,000
West Palm Beach Florida | Minimum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss	3,300,000
Winter Haven Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Remediation Expense	425,000
Seaford | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss	500,000
Seaford | Minimum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss	200,000
FPU
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	5,900,000	8,000,000.0
Approval of recovery of environmental costs	14,000,000.0
Environmental costs recovered	12,400,000	11,900,000
FPU | Manufactured Gas Plant
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs	$ 1,600,000	$ 2,100,000