Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 1	$ 0	$ 1	$ 1
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	1	1	3	4
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	3	13	11
Interest cost	9	8	26	26
Expected return on assets	(14)	(13)	(41)	(40)
Amortization of:
Amortization of actuarial loss (gain)	2	2	5	4
Settlement cost	0	0	0	2
Net periodic benefit cost	1	0	3	3
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	0	1	0
Interest cost	2	2	5	6
Expected return on assets	0	0	0	0
Amortization of:
Amortization of actuarial loss (gain)	(1)	0	(2)	(1)
Settlement cost	0	0	0	0
Net periodic benefit cost	$ 2	$ 2	$ 4	$ 5