Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 3	$ 3
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	2	1
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5
Interest cost	7	7
Expected return on assets	(12)	(12)
Amortization of:
Prior service (benefit) cost	0	0
Actuarial (gain) loss	5	4
Settlement cost	0	7	[1]
Net periodic benefit cost	5	11
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	2	2
Expected return on assets	0	0
Amortization of:
Prior service (benefit) cost	(1)	(1)
Actuarial (gain) loss	0	0
Settlement cost	0	0
Net periodic benefit cost	$ 2	$ 2

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