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, 2019		Dec. 31, 2018		Dec. 31, 2017
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 12		$ 16		$ 14
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	7		8		6
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20		21		20
Interest cost	31		29		31
Expected return on plan assets	(51)		(49)		(48)
Amortization of:
Actuarial loss	0		19		17
Prior service (benefit) cost	16		0		0
Settlement loss	1		2		7
Net periodic benefit cost	17		22		27
New prior service cost	0		0		0
Net loss (gain) arising during the year (includes curtailment gain)	(17)		62		(5)
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)	(17)		(20)		(24)
Total recognized in OCI and regulatory assets	(34)		42		(29)
Total recognized in net periodic benefit cost, OCI and regulatory assets	(17)		64		(2)
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	[1]	2	[1]	2
Interest cost	7	[1]	7	[1]	7
Expected return on plan assets	0		0		0
Amortization of:
Actuarial loss	1		1		0
Prior service (benefit) cost	(2)		(2)		(2)
Settlement loss	0		0		0
Net periodic benefit cost	7		8		7
New prior service cost	0		0		0
Net loss (gain) arising during the year (includes curtailment gain)	9		(14)		22
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)		(1)		0
Total recognized in OCI and regulatory assets	10		(13)		24
Total recognized in net periodic benefit cost, OCI and regulatory assets	$ 17		$ (5)		$ 31

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