Accounting for Derivative Instruments and Hedging Activities - Fair Value of Overall Contractual Contingent Liability Positions (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Credit Derivatives [Line Items]
Fair Value Asset/ (Liability)	$ (6.6)
Derivative Exposure Asset/ (Liability)	(6.6)
Posted Collateral	0
Tampa Electric Company [Member]
Credit Derivatives [Line Items]
Fair Value Asset/ (Liability)	(6.3)
Derivative Exposure Asset/ (Liability)	(6.3)
Posted Collateral	$ 0