Other Comprehensive Income - Accumulated Other Comprehensive Loss (Income) (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015		Dec. 31, 2015
Other Comprehensive Income Loss [Line Items]
Total other comprehensive income (loss), Gross		$ (10.1)		$ 4.1		$ (8.8)		$ 11.6
Amortization of unrecognized benefit costs and other, Gross		0.8	[1]	0.4	[2]	1.3	[1]	2.9	[2]
Change in benefit obligation due to remeasurement, Gross	[3]	(11.2)				(11.2)
Recognized cost due to curtailment , Gross	[2]	0.0				0.1
Change in benefit obligation due to valuation, Gross	[4]			(8.7)				(8.7)
Recognized cost due to settlement, Gross	[5]			12.1				12.1
Total other comprehensive income (loss), Tax		3.9		(1.7)		3.4		(4.5)
Amortization of unrecognized benefit costs and other, Tax	[1]	(0.3)		(0.2)		(0.5)		(1.1)
Change in benefit obligation due to remeasurement, Tax	[3]	4.3				4.3
Recognized cost due to curtailment , Tax	[2]	0.0				0.0
Change in benefit obligation due to valuation, Tax	[4]			3.0				3.0
Recognized cost due to settlement, Tax	[5]			(4.4)				(4.4)
Other comprehensive income (loss), net of tax		(6.2)		2.4		(5.4)		7.1
Amortization of unrecognized benefit costs and other, Net	[1]	0.5		0.2		0.8		1.8
Change in benefit obligation due to remeasurement, Net	[3]	(6.9)				(6.9)
Recognized cost due to curtailment , Net		0.0	[2]	0.0		0.1	[2]	0.0
Change in benefit obligation due to valuation, Net	[4]			(5.7)				(5.7)
Recognized cost due to settlement, Net		0.0		7.7	[5]	0.0		7.7	[5]
Unamortized pension loss and prior service credit	[6]	(37.7)				(37.7)				$ (34.2)
Unamortized other benefit gains, prior service costs and transition obligations	[7]	23.1				23.1				25.6
Net unrealized losses from cash flow hedges	[8]	(3.0)				(3.0)				(3.6)
Total accumulated other comprehensive loss		(17.6)				(17.6)				(12.2)
Tampa Electric Company [Member]
Other Comprehensive Income Loss [Line Items]
Total other comprehensive income (loss), Gross		0.3		0.3		1.0		5.3
Total other comprehensive income (loss), Tax		(0.1)		(0.1)		(0.4)		(2.0)
Other comprehensive income (loss), net of tax		0.2		0.2		0.6		3.3
Net unrealized losses from cash flow hedges	[8]	(3.0)				(3.0)				(3.6)
Total accumulated other comprehensive loss		(3.0)				(3.0)				$ (3.6)
Accumulated Gain on Cash Flow Hedges [Member]
Other Comprehensive Income Loss [Line Items]
Unrealized gain on cash flow hedges, Gross		0.0		0.0		0.0		4.3
Reclassification from AOCI to net income, Gross	[9]	0.3		0.3		1.0		1.0
Total other comprehensive income (loss), Gross		0.3		0.3		1.0		5.3
Unrealized gain on cash flow hedges, Tax		0.0		0.0		0.0		(1.5)
Reclassification from AOCI to net income, Tax	[9]	(0.1)		(0.1)		(0.4)		(0.5)
Total other comprehensive income (loss), Tax		(0.1)		(0.1)		(0.4)		(2.0)
Unrealized gain on cash flow hedges, Net		0.0		0.0		0.0		2.8
Reclassification from AOCI to net income, Net	[9]	0.2		0.2		0.6		0.5
Other comprehensive income (loss), net of tax		0.2		0.2		0.6		3.3
Accumulated Gain on Cash Flow Hedges [Member] | Tampa Electric Company [Member]
Other Comprehensive Income Loss [Line Items]
Unrealized gain on cash flow hedges, Gross		0.0		0.0		0.0		4.3
Reclassification from AOCI to net income, Gross		0.3		0.3		1.0		1.0
Total other comprehensive income (loss), Gross		0.3		0.3		1.0		5.3
Unrealized gain on cash flow hedges, Tax		0.0		0.0		0.0		(1.5)
Reclassification from AOCI to net income, Tax		(0.1)		(0.1)		(0.4)		(0.5)
Total other comprehensive income (loss), Tax		(0.1)		(0.1)		(0.4)		(2.0)
Unrealized gain on cash flow hedges, Net		0.0		0.0		0.0		2.8
Reclassification from AOCI to net income, Net		0.2		0.2		0.6		0.5
Other comprehensive income (loss), net of tax		$ 0.2		$ 0.2		$ 0.6		$ 3.3

[1] Related to postretirement benefits. See Note 5 for additional information.

[2] Related to the estimated curtailment loss for the SERP. See Note 5 for additional information.

[3] Related to remeasurement of employee postretirement benefit plans on the Merger closing date. See Note 5 for additional information.

[4] Related to the transfer of employees and their associated postretirement benefits from TEC to the TECO Energy shared services company. TEC recognized these deferred costs as regulatory assets, whereas the shared services company recognized them in AOCI.

[5] Related to the settlement of the TECO Coal black lung obligation at the closing of the sale. See Notes 15 for additional information.

[6] Net of tax benefit of $23.5 million and $21.5 million as of Sept. 30, 2016 and Dec. 31, 2015, respectively.

[7] Net of tax expense of $14.3 million and $16.1 million as of Sept. 30, 2016 and Dec. 31, 2015, respectively.

[8] Net of tax benefit of $1.9 million and $2.3 million as of Sept. 30, 2016 and Dec. 31, 2015, respectively.

[9] Related to interest rate contracts recognized in Interest expense.