Accounting for Derivative Instruments and Hedging Activities - Derivative Volumes Expected to Settle (Detail)	Sep. 30, 2014 gal
Diesel Fuel Derivatives [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	500,000
Derivative, Nonmonetary Notional Amount	500,000
Diesel Fuel Derivatives [Member] | 2014 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | 2014 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	500,000
Derivative, Nonmonetary Notional Amount	500,000
Diesel Fuel Derivatives [Member] | 2015 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | 2015 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | 2016 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | 2016 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	51,400,000
Derivative, Nonmonetary Notional Amount	51,400,000
Natural Gas Contracts [Member] | Financial [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	51,400,000
Derivative, Nonmonetary Notional Amount	51,400,000
Natural Gas Contracts [Member] | 2014 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2014 [Member] | Physical [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2014 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	11,600,000
Derivative, Nonmonetary Notional Amount	11,600,000
Natural Gas Contracts [Member] | 2014 [Member] | Financial [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	11,600,000
Derivative, Nonmonetary Notional Amount	11,600,000
Natural Gas Contracts [Member] | 2015 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2015 [Member] | Physical [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2015 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	32,200,000
Derivative, Nonmonetary Notional Amount	32,200,000
Natural Gas Contracts [Member] | 2015 [Member] | Financial [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	32,200,000
Derivative, Nonmonetary Notional Amount	32,200,000
Natural Gas Contracts [Member] | 2016 [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2016 [Member] | Physical [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | 2016 [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	7,600,000
Derivative, Nonmonetary Notional Amount	7,600,000
Natural Gas Contracts [Member] | 2016 [Member] | Financial [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	7,600,000
Derivative, Nonmonetary Notional Amount	7,600,000