Employee Postretirement Benefits - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Reclassification of regulatory assets to net income as part of periodic benefit cost	$ 4,000,000	$ 3,000,000	$ 14,000,000	$ 8,000,000
TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA			6.85%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	4,000,000	3,000,000	$ 9,000,000	10,000,000
Employer contributions			8,000,000	36,000,000
Expected future employer contributions for remainder of 2018	0		0
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	5,000,000	5,000,000	$ 17,000,000	22,000,000
Discount rate			3.63%
Employer contributions			$ 10,000,000	46,000,000
Expected future employer contributions for remainder of 2018	0		0
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	2,000,000	1,000,000	6,000,000	4,000,000
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 2,000,000	$ 1,000,000	$ 6,000,000	$ 5,000,000
Discount rate			3.70%