Employee Postretirement Benefits - Schedule of Change in Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets
Employer contributions	$ 30.9	$ 43.9
TECO Energy [Member]
Change in plan assets
Fair value of plan assets at beginning of year	625.4
Fair value of plan assets at end of year	649.4	625.4
TECO Energy [Member] | Pension Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	625.4	648.0
Actual return on plan assets	55.3	(25.5)
Employer contributions	37.4	55.0
Employer direct benefit payments	2.9	0.9
Plan participants’ contributions	0.0	0.0
Plan settlement	(2.1)	0.0
Benefits paid	(68.7)	(53.0)
Direct benefit payments	(0.8)	0.0
Fair value of plan assets at end of year	649.4	625.4
TECO Energy [Member] | Other Postretirement Benefits [Member]
Change in plan assets
Fair value of plan assets at beginning of year	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	(2.6)	(2.1)
Employer direct benefit payments	13.9	13.4
Plan participants’ contributions	2.6	2.1
Plan settlement	0.0	0.0
Benefits paid	(13.9)	(13.4)
Direct benefit payments	0.0	0.0
Fair value of plan assets at end of year	$ 0.0	$ 0.0