Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 4
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	7	8
Expected return on assets	(13)	(12)
Amortization of:
Actuarial (gain) loss	5	3
Settlement cost	0	1
Net periodic benefit cost	4	5
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	1	2
Expected return on assets	0	0
Amortization of:
Actuarial (gain) loss	0	0
Settlement cost	0	0
Net periodic benefit cost	$ 1	$ 2