Employee Postretirement Benefits - Schedule of Change in Plan Assets (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Change in plan assets
Employer contributions		$ 36		$ 31
TECO Energy [Member]
Change in plan assets
Fair value of plan assets at beginning of year		649
Fair value of plan assets at end of year		766		649
TECO Energy [Member] | Pension Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year		649	[1]	625
Actual return on plan assets		122		55
Employer contributions		46		38
Employer direct benefit payments		27		3
Plan participants’ contributions		0		0
Plan settlement		(26)		(2)
Benefits paid		(51)		(69)
Direct benefit payments		(1)		(1)
Fair value of plan assets at end of year	[1]	766		649
TECO Energy [Member] | Other Postretirement Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	[2]	0	[1]	0
Actual return on plan assets	[2]	0		0
Employer contributions	[2]	(3)		(3)
Employer direct benefit payments	[2]	16		14
Plan participants’ contributions	[2]	3		3
Plan settlement	[2]	0		0
Benefits paid	[2]	(16)		(14)
Direct benefit payments	[2]	0		0
Fair value of plan assets at end of year	[1],[2]	$ 0		$ 0

[1] The MRV of plan assets is used as the basis for calculating the EROA component of periodic pension expense. MRV reflects the fair value of plan assets adjusted for experience gains and losses (i.e. the differences between actual investment returns and expected returns) spread over five years.

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