Environmental Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) site	Dec. 31, 2019 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site		7
Environmental liabilities		$ 4,447,000	$ 6,468,000
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		500,000
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities		6,200,000	8,000,000.0
Approval of recovery of environmental costs		14,000,000.0
Environmental costs recovered	$ 11,900,000	12,300,000
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets for future recovery of environmental costs		$ 1,700,000	$ 2,100,000