Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($) 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		$ 3,709,000	$ 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		300,000
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		5,400,000	5,900,000
Approval of recovery of environmental costs		14,000,000
Environmental costs recovered	$ 12,400,000	12,700,000
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets for future recovery of environmental costs		$ 1,300,000	$ 1,600,000