Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax liabilities
Property related	$ 1,023.3	$ 884.2
Deferred fuel	11.3	3.9
Pension	43.0	38.4
Total deferred tax liabilities	1,077.6	926.5
Deferred tax assets
Alternative minimum tax credit carryforward	211.8	196.1
Loss and credit carryforwards	473.2	503.4
Other postretirement benefits	68.0	69.5
Other	113.0	89.1
Total deferred tax assets	866.0	858.1
Valuation allowance	(3.0)	(9.7)
Total deferred tax assets, net of valuation allowance	863.0	848.4
Total deferred tax liability (asset), net	214.6	78.1
Less: Current portion of deferred tax asset	(63.3)	(72.7)
Long-term portion of deferred tax liability, net	277.9	150.8
TAMPA ELECTRIC CO [Member]
Deferred tax liabilities
Property related	1,016.2	879.1
Deferred fuel	11.3	3.9
Pension and postretirement benefits	106.6	99.0
Pension	36.7	31.7
Other	22.2	14.3
Total deferred tax liabilities	1,193.0	1,028.0
Deferred tax assets
Medical benefits	49.0	50.0
Insurance reserves	31.1	28.2
Investment tax credits	5.5	5.7
Hedging activities	5.5	2.9
Pension and postretirement benefits	106.6	99.0
Unbilled revenue	14.8	19.6
Capitalized energy conservation assistance costs	19.6	20.0
Total deferred tax assets	232.1	225.4
Total deferred tax liability (asset), net	960.9	802.6
Less: Current portion of deferred tax asset	(20.0)	(30.4)
Long-term portion of deferred tax liability, net	$ 980.9	$ 833.0