Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	$ 19.6
Future Minimum Lease and Capacity Payments, 2014	19.1
Future Minimum Lease and Capacity Payments, 2015	18.2
Future Minimum Lease and Capacity Payments, 2016	17.0
Future Minimum Lease and Capacity Payments, 2017	11.9
Future Minimum Lease and Capacity Payments, Thereafter	25.3
Total future minimum payments	111.1
TAMPA ELECTRIC CO [Member]
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	16.9
Future Minimum Lease and Capacity Payments, 2014	17.0
Future Minimum Lease and Capacity Payments, 2015	17.2
Future Minimum Lease and Capacity Payments, 2016	16.9
Future Minimum Lease and Capacity Payments, 2017	11.8
Future Minimum Lease and Capacity Payments, Thereafter	25.3
Total future minimum payments	105.1
Capacity Payments [Member]
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	14.6
Future Minimum Lease and Capacity Payments, 2014	14.7
Future Minimum Lease and Capacity Payments, 2015	14.9
Future Minimum Lease and Capacity Payments, 2016	14.6
Future Minimum Lease and Capacity Payments, 2017	9.9
Future Minimum Lease and Capacity Payments, Thereafter	10.1
Total future minimum payments	78.8
Capacity Payments [Member] | TAMPA ELECTRIC CO [Member]
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	14.6
Future Minimum Lease and Capacity Payments, 2014	14.7
Future Minimum Lease and Capacity Payments, 2015	14.9
Future Minimum Lease and Capacity Payments, 2016	14.6
Future Minimum Lease and Capacity Payments, 2017	9.9
Future Minimum Lease and Capacity Payments, Thereafter	10.1
Total future minimum payments	78.8
Operating Leases [Member]
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	5.0
Future Minimum Lease and Capacity Payments, 2014	4.4
Future Minimum Lease and Capacity Payments, 2015	3.3
Future Minimum Lease and Capacity Payments, 2016	2.4
Future Minimum Lease and Capacity Payments, 2017	2.0
Future Minimum Lease and Capacity Payments, Thereafter	15.2
Total future minimum payments	32.3
Operating Leases [Member] | TAMPA ELECTRIC CO [Member]
Sale Leaseback Transaction [Line Items]
Future Minimum Lease and Capacity Payments, 2013	2.3
Future Minimum Lease and Capacity Payments, 2014	2.3
Future Minimum Lease and Capacity Payments, 2015	2.3
Future Minimum Lease and Capacity Payments, 2016	2.3
Future Minimum Lease and Capacity Payments, 2017	1.9
Future Minimum Lease and Capacity Payments, Thereafter	15.2
Total future minimum payments	$ 26.3