Employee Postretirement Benefits - Schedule of Change in Plan Assets (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Change in plan assets
Employer contributions		$ 10		$ 10		$ 15
TECO Energy [Member]
Change in plan assets
Fair value of plan assets at beginning of year		686
Fair value of plan assets at end of year		686		686
TECO Energy [Member] | Pension Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year		686	[1]	650
Actual gain (loss) return on plan assets		41		78
Employer contributions		16		16
Employer direct benefit payments		0		7
Plan participants’ contributions		0		0
Benefits paid		(57)		(58)
Direct benefit payments		0		(1)
Plan settlement	[2]	0		(6)
Fair value of plan assets at end of year		686	[1]	686	[1]	650
TECO Energy [Member] | Other Postretirement Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	[3]	0	[1]	0
Actual gain (loss) return on plan assets	[3]	0		0
Employer contributions	[3]	0		0
Employer direct benefit payments	[3]	6		15
Plan participants’ contributions	[3]	4		4
Benefits paid	[3]	0		0
Direct benefit payments	[3]	(10)		(19)
Plan settlement	[2],[3]	0		0
Fair value of plan assets at end of year	[3]	$ 0	[1]	$ 0	[1]	$ 0

[1] The market-related value of plan assets is used as the basis for calculating the expected return on plan assets component of periodic pension expense. The market-related value reflects the fair value of plan assets adjusted for experience gains and losses (i.e. the differences between actual investment returns and expected returns) spread over five years.

[2] 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.

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