Accounting for Derivative Instruments and Hedging Activities - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$ 200,000	$ 0
Derivative liabilities	26,600,000	42,700,000
Cash collateral posted with or received from any counterparties	0	0
Net pretax gain (loss) expected to be reclassified from regulatory assets or liabilities	(23,700,000)
Tampa Electric Company [Member]
Derivative [Line Items]
Derivative assets	0	0
Derivative liabilities	26,600,000	42,700,000
Cash collateral posted with or received from any counterparties	0	$ 0
Net pretax gain (loss) expected to be reclassified from regulatory assets or liabilities	$ (23,900,000)
Natural Gas Contracts [Member]
Derivative [Line Items]
Maximum length of time hedging in future cash flow	Sep. 30, 2017
Natural Gas Contracts [Member] | Tampa Electric Company [Member]
Derivative [Line Items]
Maximum length of time hedging in future cash flow	Sep. 30, 2017