Employee Postretirement Benefits - Schedule of Amounts Recognized in Net Periodic Benefit Cost, OCI, and Regulatory Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 16		$ 14		$ 13
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	8		6		6
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	21		20		19
Interest cost	29		31		31
Expected return on plan assets	(49)		(48)		(46)
Amortization of:
Actuarial loss	19		17		16
Prior service (benefit) cost	0		0		0
Curtailment loss (gain)	0		0		1
Settlement loss	2		7		1
Net periodic benefit cost	22		27		22
New prior service cost	0		0		1
Net loss (gain) arising during the year	62		(5)		47
Amounts recognized as component of net periodic benefit cost:
Amortization or curtailment recognition of prior service (benefit) cost	0		0		0
Amortization or settlement of actuarial gain (loss)	(20)		(24)		(17)
Total recognized in OCI and regulatory assets	42		(29)		31
Total recognized in net periodic benefit cost, OCI and regulatory assets	64		(2)		53
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	[1]	2	[1]	2
Interest cost	7	[1]	7	[1]	7
Expected return on plan assets	0		0		0
Amortization of:
Actuarial loss	1		0		0
Prior service (benefit) cost	(2)		(2)		(2)
Curtailment loss (gain)	0		0		0
Settlement loss	0		0		0
Net periodic benefit cost	8		7		7
New prior service cost	0		0		0
Net loss (gain) arising during the year	(14)		22		5
Amounts recognized as component of net periodic benefit cost:
Amortization or curtailment recognition of prior service (benefit) cost	2		2		2
Amortization or settlement of actuarial gain (loss)	(1)		0		0
Total recognized in OCI and regulatory assets	(13)		24		7
Total recognized in net periodic benefit cost, OCI and regulatory assets	$ (5)		$ 31		$ 14

[1] Represent amounts for TECO Energy’s Florida-based other postretirement benefit plan.