Accounting for Derivative Instruments and Hedging Activities - Effect of Hedging Instruments on Other Comprehensive Income and Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	$ (4.6)	$ 1.2	$ 0.6
Gain/(Loss) Reclassified From AOCI Into Income	(0.4)	2.0	(2.5)
TAMPA ELECTRIC CO [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(Loss) Reclassified From AOCI Into Income	(0.8)	(0.7)	(0.8)
Diesel Fuel Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	0.3	1.2	0.6
Location of Gain/(Loss) Reclassified From AOCI Into Income	Mining related costs	Mining related costs	Mining related costs
Gain/(Loss) Reclassified From AOCI Into Income	0.4	2.7	(0.8)
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain/(Loss) on Derivatives Recognized in OCI	(4.9)	0	0
Location of Gain/(Loss) Reclassified From AOCI Into Income	Interest expense	Interest expense	Interest expense
Gain/(Loss) Reclassified From AOCI Into Income	(0.8)	(0.7)	(1.7)
Interest rate contracts [Member] | TAMPA ELECTRIC CO [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Location of Gain/(Loss) Reclassified From AOCI Into Income	Interest expense
Gain/(Loss) Reclassified From AOCI Into Income	$ (0.8)	$ (0.7)	$ (0.8)