Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2021 USD ($) site	Dec. 31, 2020 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site	7
Environmental liabilities	$ 4,052,000	$ 4,299,000
West Palm Beach Florida [Member] | Minimum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	3,300,000
West Palm Beach Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	14,200,000
Winter Haven Florida [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense	400,000
Seaford [Member] | Minimum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	200,000
Seaford [Member] | Maximum [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	900,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	5,700,000	5,900,000
Approval of recovery of environmental costs	14,000,000.0
Environmental costs recovered	12,500,000	12,400,000
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets for future recovery of environmental costs	$ 1,500,000	$ 1,600,000