Variable Interest Entities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	$ 105.3	$ 125.9	$ 179.6
TAMPA ELECTRIC CO [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	105.3	125.9	179.6
Minimum range [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Multiple PPAs range	117
Minimum range [Member] | TAMPA ELECTRIC CO [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Multiple PPAs range	117
Maximum range [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Multiple PPAs range	370
Maximum range [Member] | TAMPA ELECTRIC CO [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Multiple PPAs range	370
Power Purchase Agreements [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	46.6	34.4	52.8
Power Purchase Agreements [Member] | TAMPA ELECTRIC CO [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	46.6	34.4	52.8
Power Purchase Agreements [Member] | Variable Interest Entities [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	75.8	81.2	108.8
Power Purchase Agreements [Member] | Variable Interest Entities [Member] | TAMPA ELECTRIC CO [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Purchased power	$ 75.8	$ 81.2	$ 108.8