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, 2017		Dec. 31, 2016		Dec. 31, 2015
Pension Benefits [Member]
Amortization of:
Net periodic benefit cost	$ 14		$ 13		$ 14
Other Postretirement Benefits [Member]
Amortization of:
Net periodic benefit cost	6		6		6
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20		19		21
Interest cost	31		31		30
Expected return on plan assets	(48)		(46)		(43)
Amortization of:
Actuarial loss	17		16		15
Prior service (benefit) cost	0		0		0
Curtailment loss (gain)	0		1		0
Settlement loss (2)	7		1		0
Net periodic benefit cost	27		22		23
New prior service cost	0		1		0
Net loss (gain) arising during the year	(5)		47		75
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)	(24)		(17)		(15)
Total recognized in OCI and regulatory assets	(29)		31		60
Total recognized in net periodic benefit cost, OCI and regulatory assets	(2)		53		83
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	0		0		0
Prior service (benefit) cost	(2)		(2)		(3)
Curtailment loss (gain)	0		0		0
Settlement loss (2)	0		0		0
Net periodic benefit cost	7		7		6
New prior service cost	0		0		0
Net loss (gain) arising during the year	22		5		0
Amounts recognized as component of net periodic benefit cost:
Amortization or curtailment recognition of prior service (benefit) cost	2		2		3
Amortization or settlement of actuarial gain (loss)	0		0		0
Total recognized in OCI and regulatory assets	24		7		3
Total recognized in net periodic benefit cost, OCI and regulatory assets	$ 31		$ 14		$ 9

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