Employee Postretirement Benefits - Schedule of Postretirement Benefit Amounts Recognized in Accumulated Other Comprehensive Income, Pretax and Regulatory Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 263.6	$ 203.7
Prior service cost (credit)	0.3	0.0
Unrecognized costs in regulated asset acquired in business combination	0.0	0.0
Amount recognized, pretax	263.9	203.7
Pension Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	208.2	167.7
Prior service cost (credit)	0.0	(0.3)
Amount recognized	208.2	167.4
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	10.9	9.6
Prior service cost (credit)	(25.0)	(23.6)
Unrecognized costs in regulated asset acquired in business combination	4.9	6.0
Amount recognized, pretax	(9.2)	(8.0)
Other Postretirement Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	47.2	39.5
Prior service cost (credit)	(17.0)	(19.1)
Amount recognized	$ 30.2	$ 20.4