Employee Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 208.2	$ 167.4
Accrued benefit costs and other current liabilities	(10.5)	(4.9)
Deferred credits and other liabilities	(97.0)	(76.0)
Accumulated other comprehensive loss (income), pretax	55.7	36.3
Net amount recognized at end of year	156.4	122.8
Pension Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	208.2	167.4
Accrued benefit costs and other current liabilities	(0.6)	(0.6)
Deferred credits and other liabilities	(69.3)	(53.5)
Net amount recognized at end of year	138.3	113.3
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	32.4	26.6
Accrued benefit costs and other current liabilities	(10.7)	(10.7)
Deferred credits and other liabilities	(163.9)	(172.0)
Accumulated other comprehensive loss (income), pretax	(41.6)	(34.6)
Net amount recognized at end of year	(183.8)	(190.7)
Other Postretirement Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	30.2	20.4
Accrued benefit costs and other current liabilities	(9.2)	(9.1)
Deferred credits and other liabilities	(142.3)	(137.1)
Net amount recognized at end of year	$ (121.3)	$ (125.8)