Accounting for Derivative Instruments and Hedging Activities - Gross Amounts of Derivatives and Their Related Offset Amounts (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets, Gross Amounts of Recognized Assets	$ 1.4	$ 10.5
Derivative assets, Gross Amounts offset on the Balance Sheet	(1.2)	(0.5)
Derivative assets, Net Amounts of Assets	0.2	10.0
Derivative liabilities, Gross Amounts of Recognized (Liabilities)	(6.9)	(0.8)
Derivative liabilities, Gross Amounts offset on the Balance Sheet	1.2	0.5
Derivative assets, Net Amounts of Liabilities	(5.7)	(0.3)
Tampa Electric Company [Member]
Derivative [Line Items]
Derivative assets, Gross Amounts of Recognized Assets	1.4	10.3
Derivative assets, Gross Amounts offset on the Balance Sheet	(1.3)	(0.5)
Derivative assets, Net Amounts of Assets	0.1	9.8
Derivative liabilities, Gross Amounts of Recognized (Liabilities)	(6.9)	(0.7)
Derivative liabilities, Gross Amounts offset on the Balance Sheet	1.3	0.5
Derivative assets, Net Amounts of Liabilities	$ (5.6)	$ (0.2)