Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Pension Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year		$ 812		$ 770
Service cost		21		20		$ 19
Interest cost		29		31		31
Plan participants’ contributions		0		0
Plan curtailment		0		(1)
Plan settlement		(7)		(26)
Benefits paid		(55)		(51)
Actuarial loss (gain)		(50)		69
Net benefit obligation at end of year		750		812		770
Other Postretirement Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year	[1]	193		175
Service cost		2	[1]	2	[1]	2
Interest cost		7	[1]	7	[1]	7
Plan participants’ contributions	[1]	4		3
Plan curtailment	[1]	0		0
Plan settlement	[1]	0		0
Benefits paid	[1]	(19)		(16)
Actuarial loss (gain)	[1]	(14)		22
Net benefit obligation at end of year	[1]	$ 173		$ 193		$ 175

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