Environmental Commitments and Contingencies - Additional Information (Detail)	6 Months Ended	12 Months Ended
	Jun. 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	$ 4,520,000	$ 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	6,100,000	8,000,000.0
Approval of recovery of environmental costs	14,000,000.0
Environmental costs recovered	12,200,000	11,900,000
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets for future recovery of environmental costs	$ 1,800,000	$ 2,100,000