Other Comprehensive Income - Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrealized gain (loss) on cash flow hedges, Gross	$ (7.4)	$ 1.8	$ 1.0
Reclassification from AOCI to net income, Gross	0.6	(3.1)	3.9
Gain (Loss) on cash flow hedges, Gross	(6.8)	(1.3)	4.9
Amortization of unrecognized benefit costs and other, Gross	(4.8)	(7.9)	3.7
Recognized benefit costs due to settlement, Gross		0.9	1.7
Total other comprehensive (loss) income, Gross	(11.6)	(8.3)	10.3
Unrealized gain (loss) on cash flow hedges, Tax	2.8	(0.6)	(0.4)
Reclassification from AOCI to net income, Tax	(0.2)	1.1	(1.4)
Gain (Loss) on cash flow hedges, Tax	2.6	0.5	(1.8)
Amortization of unrecognized benefit costs and other, Tax	0	3.3	0
Recognized benefit costs due to settlement, Tax		(0.3)	(0.7)
Total other comprehensive (loss) income, Tax	2.6	3.5	(2.5)
Unrealized gain (loss) on cash flow hedges, Net	(4.6)	1.2	0.6
Reclassification from AOCI to net income, Net	0.4	(2.0)	2.5
Gain (Loss) on cash flow hedges, Net	(4.2)	(0.8)	3.1
Amortization of unrecognized benefit costs and other, Net	(4.8)	(4.6)	3.7
Recognized benefit costs due to settlement, Net		0.6	1.0
Other comprehensive (loss) income, net of tax	(9.0)	(4.8)	7.8
Unrecognized pension losses and prior service credits	(32.9)	(31.2)
Unrecognized other benefit gains, prior service costs and transition obligations	11.1	14.2
Net unrealized losses from cash flow hedges	(9.2)	(5.0)
Total accumulated other comprehensive loss	(31.0)	(22.0)
TAMPA ELECTRIC CO [Member]
Unrealized gain (loss) on cash flow hedges, Gross	(8.0)	0	0
Reclassification from AOCI to net income, Gross	1.4	1.2	1.2
Gain (Loss) on cash flow hedges, Gross	(6.6)	1.2	1.2
Total other comprehensive (loss) income, Gross	(6.6)	1.2	1.2
Unrealized gain (loss) on cash flow hedges, Tax	3.1	0	0
Reclassification from AOCI to net income, Tax	(0.6)	(0.5)	(0.4)
Gain (Loss) on cash flow hedges, Tax	2.5	(0.5)	(0.4)
Total other comprehensive (loss) income, Tax	2.5	(0.5)	(0.4)
Unrealized gain (loss) on cash flow hedges, Net	(4.9)	0	0
Reclassification from AOCI to net income, Net	0.8	0.7	0.8
Gain (Loss) on cash flow hedges, Net	(4.1)	0.7	0.8
Other comprehensive (loss) income, net of tax	(4.1)	0.7	0.8
Net unrealized losses from cash flow hedges	(8.7)	(4.6)
Total accumulated other comprehensive loss	$ (8.7)	$ (4.6)