Employee Postretirement Benefits - Schedule of Amount Recognized in Balance Sheet (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	$ 216.5	$ 199.7
Accrued benefit costs and other current liabilities	(5.3)	(2.9)
Deferred credits and other liabilities	(180.6)	(175.9)
Net amount recognized at end of year	83.7	71.7
Pension Benefits [Member] | TAMPA ELECTRIC CO [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	216.5	199.7
Accrued benefit costs and other current liabilities	(0.9)	(1.0)
Deferred credits and other liabilities	(139.8)	(133.2)
Net amount recognized at end of year	75.8	65.5
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	59.6	52.7
Accrued benefit costs and other current liabilities	(13.1)	(13.2)
Deferred credits and other liabilities	(217.2)	(203.3)
Net amount recognized at end of year	(188.6)	(184.2)
Other Postretirement Benefits [Member] | TAMPA ELECTRIC CO [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets	59.6	52.7
Accrued benefit costs and other current liabilities	(10.6)	(10.6)
Deferred credits and other liabilities	(174.2)	(163.6)
Net amount recognized at end of year	$ (125.2)	$ (121.5)