Employee Postretirement Benefits - Schedule of Funded Status (Detail) - TECO Energy [Member] - USD ($) $ in Millions		Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 686		$ 650
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation (PBO/APBO)		678		666		$ 850
Fair value of plan assets		686	[1]	650	[1]	924
Funded status at end of year		8		(16)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation (PBO/APBO)	[2]	132		142		200
Fair value of plan assets	[2]	0	[1]	0	[1]	$ 0
Funded status at end of year	[3]	$ (132)		$ (142)

[1] The MRV of plan assets is used as the basis for calculating the EROA component of periodic pension expense. MRV 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.

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