Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2024
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 1	$ 0
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA	7.05%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0	0
Employer contributions	2	3
Pension Benefits [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 8
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	0
Discount rate	5.27%
Employer contributions	$ 4	4
Pension Benefits [Member] | TECO Energy [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 12
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	1	1
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	$ 2
Discount rate	5.28%