Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 2	$ 3	$ 5	$ 6
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	3	1	5	3
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	5	11	10
Interest cost	6	6	11	13
Expected return on assets	(13)	(12)	(26)	(25)
Amortization of:
Actuarial loss (gain)	5	5	11	10
Settlement cost			0	0
Net periodic benefit cost	4	4	7	8
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	0	1	0
Interest cost	2	2	3	3
Expected return on assets	0	0	0	0
Amortization of:
Actuarial loss (gain)	(1)	0	0	0
Settlement cost			0	0
Net periodic benefit cost	$ 2	$ 2	$ 4	$ 3