Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Pension Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year		$ 843		$ 750
Service cost		20		20		$ 21
Interest cost		26		31		29
Plan participants’ contributions		0		0
Plan curtailment		0		(10)
Plan settlement		0		(5)
Benefits paid		(54)		(49)
Actuarial loss		84		106
Benefit obligation at end of year		919		843		750
Other Postretirement Benefits [Member]
Change in benefit obligation
Benefit obligation at beginning of year	[1]	180		173
Service cost		2	[1]	1	[1]	2
Interest cost		6	[1]	7	[1]	7
Plan participants’ contributions	[1]	4		4
Plan curtailment	[1]	0		0
Plan settlement	[1]	0		0
Benefits paid	[1]	(17)		(14)
Actuarial loss	[1]	37		9
Benefit obligation at end of year	[1]	$ 212		$ 180		$ 173

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