Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 3	$ 4	$ 9	$ 14
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA			7.35%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2	4	$ 9	9
Employer contributions			15	8
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	4	5	$ 13	17
Discount rate			4.34%
Employer contributions			$ 20	10
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2	2	5	6
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 2	$ 2	$ 5	$ 6
Discount rate			4.38%