Common Stock - Schedule of Company Equity Method Investments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Schedule of Capitalization, Equity [Line Items]
Capital Stock Outstanding December 31, Shares	216.6	215.8
Common equity	$ 216.6	$ 215.8
Common equity	216.6	215.8
TAMPA ELECTRIC CO [Member]
Schedule of Capitalization, Equity [Line Items]
Capital Stock Outstanding December 31, Shares	10.0	10.0
Common equity	1,970.4	1,852.4
Issue expense	0	0
Common equity	1,970.4	1,852.4
Equity contributions	$ 118