Environmental Commitments and Contingencies - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) site	Sep. 30, 2014 USD ($)	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	$ 9,003		$ 8,923
Amount paid for funding requirements	113	$ 134
Seaford [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	273
Accrual for Environmental Loss Contingencies	239
Regulatory Assets	273
Estimated costs of remediation range, minimum	273
Estimated costs of remediation range, maximum	465
Winter Haven Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental remediation expense	443
Additional remediation costs	100
West Palm Beach Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, minimum	4,500
Estimated costs of remediation range, maximum	15,400
Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, maximum	13,000
Cost of remedy for settlements of claims	20,000
Environmental remediation expense	24
FPU [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	10,000
Approval of recovery of environmental costs	14,000
Environmental costs recovered	10,000
FPU [Member] | Sanford Florida [Member]
Environmental Commitments And Contingencies [Line Items]
Estimated costs of remediation range, maximum	$ 650
Environmental remediation expense percent	5.00%
Amount paid for funding requirements	$ 650
FPU [Member] | Manufactured Gas Plant [Member]
Environmental Commitments And Contingencies [Line Items]
Regulatory Assets	4,000
Chesapeake [Member]
Environmental Commitments And Contingencies [Line Items]
Environmental liabilities	389
Regulatory Assets	$ 116