Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 4	$ 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	8	7
Expected return on assets	(12)	(12)
Amortization of:
Prior service (benefit) cost	0	0
Actuarial loss	3	5
Settlement cost	1	0
Net periodic benefit cost	5	5
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1
Interest cost	2	2
Expected return on assets	0	0
Amortization of:
Prior service (benefit) cost	0	(1)
Actuarial loss	0	0
Settlement cost	0	0
Net periodic benefit cost	$ 2	$ 2