Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 6	$ 7	$ 9
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	1	2	3	4
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	6	10	11
Interest cost	8	7	16	14
Expected return on assets	(14)	(12)	(26)	(24)
Amortization of:
Prior service (benefit) cost	0	0
Actuarial (gain) loss	5	4	8	9
Settlement cost	0	2	1	2
Net periodic benefit cost	4	7	9	12
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0	1
Interest cost	1	2	3	4
Expected return on assets	0	0	0	0
Amortization of:
Prior service (benefit) cost	0	1
Actuarial (gain) loss	0	(1)	0	(1)
Settlement cost	0	0	0	0
Net periodic benefit cost	$ 1	$ 2	$ 3	$ 4