Employee Postretirement Benefits - Schedule of Change in Benefit Obligation (Detail) - TECO Energy [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year	$ 732.9	$ 728.9
Service cost	18.8	20.9	$ 18.3
Interest cost	30.8	30.3	32.0
Plan participants’ contributions	0.0	0.0
Plan amendments	1.2	0.0
Plan curtailment	1.3	0.0
Plan settlement	(2.1)	0.0
Benefits paid	(69.5)	(53.0)
Actuarial loss (gain)	56.3	5.8
Net benefit obligation at end of year	769.7	732.9	728.9
Other Postretirement Benefits [Member]
Change in benefit obligation
Net benefit obligation at beginning of year	172.3	174.3
Service cost	1.8	1.9	2.4
Interest cost	7.4	7.0	10.4
Plan participants’ contributions	2.6	2.1
Plan amendments	0.0	0.0
Plan curtailment	0.0	0.0
Plan settlement	0.0	0.0
Benefits paid	(13.9)	(13.4)
Actuarial loss (gain)	5.0	0.4
Net benefit obligation at end of year	$ 175.2	$ 172.3	$ 174.3