Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 3	$ 4	$ 6	$ 8
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	3	6	$ 7	9
Employer contributions			5	8
Expected future employer contributions for remainder of 2019	10		10
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	4	7	$ 9	12
Discount rate			4.34%
Employer contributions			$ 7	10
Expected future employer contributions for remainder of 2019	14		14
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	1	2	3	4
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	$ 2	$ 3	$ 4
Discount rate			4.38%