Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 3
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	2	2
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5
Interest cost	5	7
Expected return on assets	(13)	(13)
Amortization of:
Actuarial loss (gain)	6	5
Net periodic benefit cost	3	4
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	1	1
Expected return on assets	0	0
Amortization of:
Actuarial loss (gain)	1	0
Net periodic benefit cost	$ 2	$ 1