Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 01, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 5	$ 5
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		4	3	7	6
Employer contributions				20	13
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		6	7	17	12
Employer contributions				25	16
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		2	2	3	3
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 2	$ 2	$ 4	$ 4
Discount rate	4.28%