Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 2	$ 9	$ 9
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	2	2	5	5
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	15	15
Interest cost	7	8	20	24
Expected return on assets	(13)	(12)	(38)	(38)
Amortization of:
Actuarial loss (gain)	5	3	15	11
Settlement cost			0	1
Net periodic benefit cost	4	4	12	13
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1	0	1
Interest cost	1	3	4	6
Expected return on assets	0	0	0	0
Amortization of:
Actuarial loss (gain)	0	(2)	0	(2)
Settlement cost			0	0
Net periodic benefit cost	$ 1	$ 2	$ 4	$ 5