Employee Postretirement Benefits - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 4,000,000	$ 3,000,000
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA	7.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 3,000,000	4,000,000
Employer contributions	4,000,000	0
Employer contributions in remainder of year	12,000,000
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 4,000,000	5,000,000
Discount rate	3.22%
Employer contributions	$ 5,000,000	0
Employer contributions in remainder of year	14,000,000
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2,000,000	2,000,000
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1,000,000	$ 2,000,000
Discount rate		3.32%