Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Pension Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year		$ 750		$ 812
Service cost		20		21		$ 20
Interest cost		31		29		31
Plan participants’ contributions		0		0
Plan curtailment		(10)		0
Plan settlement		(5)		(7)
Benefits paid		(49)		(55)
Actuarial loss (gain)		106		(50)
Benefit obligation at end of year		843		750		812
Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	[1]	173		193
Service cost		1	[1]	2	[1]	2
Interest cost		7	[1]	7	[1]	7
Plan participants’ contributions	[1]	4		4
Plan curtailment	[1]	0		0
Plan settlement	[1]	0		0
Benefits paid	[1]	(14)		(19)
Actuarial loss (gain)	[1]	9		(14)
Benefit obligation at end of year	[1]	$ 180		$ 173		$ 193

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