Accounting for Derivative Instruments and Hedging Activities - Derivatives Designated as Cash Flow Hedges (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current assets	$ 0	$ 0.9
Long-term assets	0.2	0
Total assets	0.2	0.9
Current liabilities	14.6	58.4
Long-term liabilities	0.6	8.6
Total liabilities	15.2	67.0
TAMPA ELECTRIC CO [Member]
Derivatives, Fair Value [Line Items]
Long-term assets	0.2	0
Current liabilities	14.1	58.4
Long-term liabilities	0.2	7.4
Diesel Fuel Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current assets	0	0.9
Long-term assets	0	0
Total assets	0	0.9
Current liabilities	0.5	0
Long-term liabilities	0.4	1.2
Total liabilities	0.9	1.2
Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current assets	0	0
Long-term assets	0.2	0
Total assets	0.2	0
Current liabilities	14.1	58.4
Long-term liabilities	0.2	7.4
Total liabilities	14.3	65.8
Natural Gas Derivatives [Member] | TAMPA ELECTRIC CO [Member]
Derivatives, Fair Value [Line Items]
Current assets	0	0
Long-term assets	0.2	0
Total assets	0.2	0
Current liabilities	14.1	58.4
Long-term liabilities	0.2	7.4
Total liabilities	$ 14.3	$ 65.8