Asset Retirement Obligations - Schedule of Asset Retirement Obligations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation [Line Items]
Beginning balance	$ 53.8	$ 55.7
Additional liabilities	0.7	0.8
Liabilities settled	(29.1)	(3.6)
Accretion expense	1.4	1.4	1.4
Revisions to estimated cash flows	0	(2.2)
Other	1.8	1.7
Ending balance	28.6	53.8	55.7
TAMPA ELECTRIC CO [Member]
Asset Retirement Obligation [Line Items]
Beginning balance	30.8	31.3
Liabilities settled	(27.6)	0
Revisions to estimated cash flows	0	(2.2)
Other	1.8	1.7
Ending balance	$ 5.0	$ 30.8