Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 5	$ 4	$ 11	$ 9
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA			6.70%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2	3	$ 5	6
Employer contributions			9	8
Employer contributions in remainder of year	8		8
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	4	4	$ 7	8
Discount rate			2.38%
Employer contributions			$ 11	10
Employer contributions in remainder of year	10		10
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	3	1	5	3
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 2	$ 2	$ 4	$ 3
Discount rate			2.47%