Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Pension Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year		$ 770		$ 733
Service cost		20		19		$ 21
Interest cost		31		31		30
Plan participants’ contributions		0		0
Plan amendments		0		1
Plan curtailment		(1)		1
Plan settlement		(26)		(2)
Benefits paid		(51)		(69)
Actuarial loss (gain)		69		56
Net benefit obligation at end of year		812		770		733
Other Postretirement Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year	[1]	175		172
Service cost		2	[1]	2	[1]	2
Interest cost		7	[1]	7	[1]	7
Plan participants’ contributions	[1]	3		3
Plan amendments	[1]	0		0
Plan curtailment	[1]	0		0
Plan settlement	[1]	0		0
Benefits paid	[1]	(16)		(14)
Actuarial loss (gain)	[1]	22		5
Net benefit obligation at end of year	[1]	$ 193		$ 175		$ 172

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