Environmental Commitments and Contingencies - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) claim site	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Environmental Commitments And Contingencies [Line Items]
Company's exposure in number of former Manufactured Gas Plant Sites | site		7
Environmental liabilities		$ 8,592,000	$ 8,942,000
Amount paid for funding requirements		350,000	$ 174,000	$ 233,000
Proceeds from Legal Settlements	$ 500,000
West Palm Beach Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		15,400,000
West Palm Beach Florida | Minimum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		4,500,000
Sanford Florida
Environmental Commitments And Contingencies [Line Items]
Cost of remedy for settlements of claims		$ 20,000,000
Loss Contingency, Pending Claims, Number | claim		2
Environmental Remediation Expense		$ 24,000
Costs to resolve liability		650,000
Sanford Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		13,000,000
Winter Haven Florida
Environmental Commitments And Contingencies [Line Items]
Environmental Remediation Expense		425,000
Additional remediation costs		100,000
Seaford | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		465,000
Seaford | Minimum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		$ 273,000
Aspire Energy
Environmental Commitments And Contingencies [Line Items]
Number Of Environmental Sites | site		8
Business Combination, Indemnification Assets, Range of Outcomes, Value, High		$ 1,600,000
Minimum Environmental Remediation Expenditures		431,250
FPU
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities		9,800,000
Approval of recovery of environmental costs		14,000,000
Environmental costs recovered		10,600,000
FPU | Manufactured Gas Plant
Environmental Commitments And Contingencies [Line Items]
Regulatory assets for future recovery of environmental costs		$ 3,400,000
FPU | Sanford Florida
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense percent		5.00%
Amount paid for funding requirements		$ 650,000
FPU | Sanford Florida | Maximum
Environmental Commitments And Contingencies [Line Items]
Environmental Exit Costs, Reasonably Possible Additional Loss		$ 650,000