Accounting for Derivative Instruments and Hedging Activities - Energy Related Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Regulatory liabilities	$ 65.8	$ 85.8
Regulatory liabilities	727.2	631.4
Regulatory liabilities	793.0	717.2
Regulatory assets	24.3	34.3
Long-term regulatory assets	342.2	293.1
Regulatory assets	366.5	327.4
Regulatory assets	342.2	293.1
Tampa Electric Company [Member]
Derivatives Fair Value [Line Items]
Regulatory liabilities	64.5	85.8
Regulatory liabilities	623.5	631.4
Regulatory liabilities	688.0	717.2
Regulatory assets	22.9	34.3
Long-term regulatory assets	311.7	293.1
Regulatory assets	334.6	327.4
Regulatory assets	311.7	293.1
Energy Related Derivatives [Member]
Derivatives Fair Value [Line Items]
Regulatory liabilities	0.1	9.5
Regulatory liabilities	0	0.3
Regulatory liabilities	0.1	9.8
Regulatory assets	4.0	0
Long-term regulatory assets	1.6	0.2
Regulatory assets	5.6	0.2
Regulatory assets	1.6	0.2
Energy Related Derivatives [Member] | Tampa Electric Company [Member]
Derivatives Fair Value [Line Items]
Regulatory liabilities	0.1	9.5
Regulatory liabilities	0	0.3
Regulatory liabilities	0.1	9.8
Regulatory assets	4.0	0
Long-term regulatory assets	1.6	0.2
Regulatory assets	5.6	0.2
Regulatory assets	$ 1.6	$ 0.2