Employee Postretirement Benefits - Schedule of Funded Status (Detail) - TECO Energy [Member] - USD ($) $ in Millions		Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 686		$ 686
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation (PBO/APBO)		674		678		$ 666
Fair value of plan assets		686	[1]	686	[1]	650
Funded status at end of year		12		8
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation (PBO/APBO)	[2]	130		132		142
Fair value of plan assets	[2]	0	[1]	0	[1]	$ 0
Funded status at end of year	[2]	$ (130)		$ (132)

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