Accounting for Derivative Instruments and Hedging Activities - Energy Related Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current regulatory assets	$ 70.3	$ 87.3
Long-term regulatory assets	382.6	364.5
Regulatory assets	452.9	451.8
Current regulatory liabilities	106.7	86.2
Long-term regulatory liabilities	651.9	619.4
Regulatory liabilities	758.6	705.6
TAMPA ELECTRIC CO [Member]
Derivatives, Fair Value [Line Items]
Current regulatory assets	70.3	87.3
Long-term regulatory assets	382.6	364.5
Regulatory assets	452.9	451.8
Current regulatory liabilities	106.7	86.2
Long-term regulatory liabilities	651.9	619.4
Regulatory liabilities	758.6	705.6
Energy Related Derivatives [Member]
Derivatives, Fair Value [Line Items]
Current regulatory assets	14.1	58.4
Long-term regulatory assets	0.2	7.4
Regulatory assets	14.3	65.8
Current regulatory liabilities	0	0
Long-term regulatory liabilities	0.2	0
Regulatory liabilities	0.2	0
Energy Related Derivatives [Member] | TAMPA ELECTRIC CO [Member]
Derivatives, Fair Value [Line Items]
Current regulatory assets	14.1	58.4
Long-term regulatory assets	0.2	7.4
Regulatory assets	14.3	65.8
Current regulatory liabilities	0	0
Long-term regulatory liabilities	0.2	0
Regulatory liabilities	$ 0.2	$ 0