Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 2	$ 4	$ 9	$ 9
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	2	2	5	6
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	15	16
Interest cost	8	8	24	22
Expected return on assets	(12)	(13)	(38)	(37)
Amortization of:
Actuarial (gain) loss	3	5	11	14
Settlement cost			1	2
Net periodic benefit cost	4	5	13	17
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	0	1	1
Interest cost	3	1	6	5
Expected return on assets	0	0	0	0
Amortization of:
Actuarial (gain) loss	(2)	1	(2)	0
Settlement cost			0	0
Net periodic benefit cost	$ 2	$ 2	$ 5	$ 6