Accounting for Derivative Instruments and Hedging Activities - Derivatives Designated as Cash Flow Hedges (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Current assets	$ 0.2	$ 0
Long-term assets	0	0.2
Total assets	0.2	0.2
Current liabilities	5.2	14.6
Long-term liabilities	1.5	0.6
Total liabilities	6.7	15.2
Tampa Electric Company [Member]
Derivatives, Fair Value [Line Items]
Current assets	0.1	0
Long-term assets	0	0.2
Current liabilities	4.9	14.1
Long-term liabilities	1.4	0.2
Diesel Fuel Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current assets	0.1	0
Long-term assets	0	0
Total assets	0.1	0
Current liabilities	0.3	0.5
Long-term liabilities	0.1	0.4
Total liabilities	0.4	0.9
Natural Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current assets	0.1	0
Long-term assets	0	0.2
Total assets	0.1	0.2
Current liabilities	4.9	14.1
Long-term liabilities	1.4	0.2
Total liabilities	6.3	14.3
Natural Gas Derivatives [Member] | Tampa Electric Company [Member]
Derivatives, Fair Value [Line Items]
Current assets	0.1	0
Long-term assets	0	0.2
Total assets	0.1	0.2
Current liabilities	4.9	14.1
Long-term liabilities	1.4	0.2
Total liabilities	$ 6.3	$ 14.3