Accounting for Derivative Instruments and Hedging Activities - Derivative Volumes Expected to Settle (Detail)	Sep. 30, 2013 gal
Diesel Fuel Derivatives [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | Physical [Member] | 2013 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | Physical [Member] | 2014 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | Physical [Member] | 2015 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Diesel Fuel Derivatives [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	3,800,000
Diesel Fuel Derivatives [Member] | Financial [Member] | 2013 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	1,800,000
Diesel Fuel Derivatives [Member] | Financial [Member] | 2014 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	2,000,000
Diesel Fuel Derivatives [Member] | Financial [Member] | 2015 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2013 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2013 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2014 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2014 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2015 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Physical [Member] | 2015 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Natural Gas Contracts [Member] | Financial [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	50,700,000
Natural Gas Contracts [Member] | Financial [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	50,700,000
Natural Gas Contracts [Member] | Financial [Member] | 2013 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	9,800,000
Natural Gas Contracts [Member] | Financial [Member] | 2013 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	9,800,000
Natural Gas Contracts [Member] | Financial [Member] | 2014 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	35,000,000
Natural Gas Contracts [Member] | Financial [Member] | 2014 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	35,000,000
Natural Gas Contracts [Member] | Financial [Member] | 2015 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	5,900,000
Natural Gas Contracts [Member] | Financial [Member] | 2015 [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	5,900,000