Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	$ 66.7		$ 66.7		$ 86.6
Franchise fees and gross receipts taxes	31.7	$ 31.7	88.3	$ 86.7
Tampa Electric Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	60.0		60.0		$ 49.3
Franchise fees and gross receipts taxes	$ 31.7	$ 31.7	$ 88.3	$ 86.7