Accounting for Derivative Instruments and Hedging Activities - Derivatives Designated as Cash Flow Hedges (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Derivative assets	$ 0.2	$ 9.7
Derivative assets	0	0.3
Total assets	0.2	10.0
Derivative liabilities	4.1	0.1
Long-term liabilities	1.6	0.2
Total liabilities	5.7	0.3
Derivative liabilities	1.6	0.2
Tampa Electric Company [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	9.5
Derivative assets	0	0.3
Total assets	0.1	9.8
Derivative liabilities	4.0	0
Long-term liabilities	1.6	0.2
Total liabilities	5.6	0.2
Derivative liabilities	1.6	0.2
Tampa Electric Company [Member] | Natural Gas Derivatives [Member]
Derivatives Fair Value [Line Items]
Derivative assets	0.1	9.5
Derivative assets	0	0.3
Total assets	0.1	9.8
Derivative liabilities	4.0	0
Long-term liabilities	1.6	0.2
Total liabilities	5.6	0.2
Derivative liabilities	$ 1.6	$ 0.2