Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Taxes [Line Items]
Unrecognized tax benefits	$ 9		$ 9
Unrecognized tax benefit that would reduce effective tax rate	9		$ 9
Tampa Electric [Member]
Income Taxes [Line Items]
Deferred employer payroll taxes, CARES Act	7
Decrease in taxes other than income for employee retention payroll tax credit	$ 3
Effective tax rate	16.70%	17.50%