Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 3	$ 4
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA	7.15%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 4	3
Employer contributions	0	8
Expected future employer contributions for remainder of 2019	15
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 5	5
Discount rate	4.34%
Employer contributions	$ 0	10
Expected future employer contributions for remainder of 2019	20
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2	2
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 2	$ 2
Discount rate	4.38%