Accounting For Derivative Instruments And Hedging Activities (Effect of Hedging Instruments On OCI and Income) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Derivatives, Fair Value [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	$ (1.3)	[1]	$ 0.7	[1]	$ 0.6	[1]	$ (0.3)	[1]
Amount of Gain/(Loss) Reclassified From AOCI Into Income Effective Portion	0.5	[1]	(0.7)	[1]	1.5	[1]	(2.1)	[1]
Tampa Electric Co [Member] | Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Location of Gain/(Loss) Reclassified From AOCI Into Income	Interest expense		Interest expense		Interest expense		Interest expense
Amount of Gain/(Loss) Reclassified From AOCI Into Income Effective Portion	(0.2)		(0.2)		(0.5)		(0.6)
Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	0	[1]	0	[1]	0	[1]	(0.1)	[1]
Location of Gain/(Loss) Reclassified From AOCI Into Income	Interest expense		Interest expense		Interest expense		Interest expense
Amount of Gain/(Loss) Reclassified From AOCI Into Income Effective Portion	(0.2)	[1]	(0.4)	[1]	(0.5)	[1]	(1.2)	[1]
Tampa Electric Co [Member]
Derivatives, Fair Value [Line Items]
Amount of Gain/(Loss) Reclassified From AOCI Into Income Effective Portion	(0.2)		(0.2)		(0.5)		(0.6)
Diesel Fuel Derivatives [Member]
Derivatives, Fair Value [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	(1.3)	[1]	0.7	[1]	0.6	[1]	(0.2)	[1]
Location of Gain/(Loss) Reclassified From AOCI Into Income	Mining related costs		Mining related costs		Mining related costs		Mining related costs
Amount of Gain/(Loss) Reclassified From AOCI Into Income Effective Portion	$ 0.7	[1]	$ (0.3)	[1]	$ 2.0	[1]	$ (0.9)	[1]

[1] Changes in OCI and AOCI are reported in after-tax dollars.