Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost		$ 3	$ 3	$ 8	$ 8
TECO Energy [Member] | Scenario Forecast [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	4	10	10
Employer contributions				36	31
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		5	5	22	17
Employer contributions				46	37
Pension Benefits [Member] | TECO Energy [Member] | Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate						4.16%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		1	2	4	5
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 1	$ 2	$ 5	$ 5
Discount rate	4.28%