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Asset Retirement Obligations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation [Line Items]
Accretion expense	$ 1.4	$ 1.4	$ 1.4
Revisions to estimated liabilities	29.1
Revisions to estimated liabilities	29.1	3.6
TAMPA ELECTRIC CO [Member]
Asset Retirement Obligation [Line Items]
Revisions to estimated liabilities	$ 27.6	$ 0

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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