Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 2	$ 3	$ 7	$ 9
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	3	2	8	5
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	5	14	15
Interest cost	5	7	16	20
Expected return on assets	(13)	(13)	(39)	(38)
Amortization of:
Actuarial loss (gain)	7	5	18	15
Settlement cost			0	0
Net periodic benefit cost	2	4	9	12
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	0	2	0
Interest cost	1	1	4	4
Expected return on assets	0	0	0	0
Amortization of:
Actuarial loss (gain)	1	0	1	0
Settlement cost			0	0
Net periodic benefit cost	$ 3	$ 1	$ 7	$ 4