Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Pension Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year		$ 666		$ 850
Service cost		15		18		$ 19
Interest cost		35		23		21
Plan participants’ contributions		0		0
Benefits paid		(59)		(79)
Actuarial loss (gain)		27		(142)
Plan amendments		0		0
Plan settlement	[1]	(6)		(4)
Benefit obligation at end of year		678		666		850
Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	[2]	142		200
Service cost	[3]	1	[2]	2	[2]	2
Interest cost	[3]	7	[2]	5	[2]	5
Plan participants’ contributions	[2]	4		4
Benefits paid	[2]	(19)		(19)
Actuarial loss (gain)	[2]	7		(50)
Plan amendments	[2]	(10)		0
Plan settlement	[1],[2]	0		0
Benefit obligation at end of year	[2]	$ 132		$ 142		$ 200

[1] Represents TECO Energy’s SERP and Restoration settlement charges as a result of the retirement of certain executives. These charges did impact TEC’s financial statements.

[2] Represent amounts for TECO Energy’s Florida-based other postretirement benefit plan.

[3] Represents amounts for TECO Energy’s Florida-based other postretirement benefit plan