Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2017	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit expense		$ 2	$ 2
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA	7.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net pension expense		3	3
Employer contributions		11	4
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.16%
Employer contributions		14	5
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net pension expense		$ 1	$ 2
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.28%