Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 4	$ 10		$ 10
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	1	2	4		5
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	15		14
Interest cost	8	7	24		23
Expected return on assets	(12)	(12)	(36)		(34)
Amortization of:
Prior service (benefit) cost	0	0	0		0
Actuarial (gain) loss	4	5	12		12
Curtailment cost			0		1
Settlement cost			7	[1]	1
Net periodic benefit cost	5	5	22		17
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	1	1		1
Interest cost	2	2	6		6
Expected return on assets	0	0	0		0
Amortization of:
Prior service (benefit) cost	0	(1)	0		(2)
Actuarial (gain) loss	(1)	0	(2)		0
Curtailment cost			0		0
Settlement cost			0		0
Net periodic benefit cost	$ 1	$ 2	$ 5		$ 5

[1] Represents TECO Energy’s SERP settlement charge as a result of retirements that occurred subsequent to the Merger with Emera.