Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.9		$ 6.7	$ 13.9	$ 17.6
Interest cost	7.2		6.5	23.6	22.6
Expected return on assets	(11.5)		(9.1)	(34.2)	(32.4)
Amortization of:
Prior service (benefit) cost	0.0		(0.1)	0.2	(0.2)
Actuarial loss	4.8		3.2	11.7	11.4
Regulatory asset	0.0		0.0	0.0	0.0
Curtailment cost	0.0	$ 1.3	0.0	1.3	0.0
Settlement cost	0.0		0.0	0.6	0.0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	5.4		7.2	17.1	19.0
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.5		0.6	1.4	1.7
Interest cost	2.0		2.0	6.4	6.1
Expected return on assets	(0.3)		(0.3)	(0.9)	(0.8)
Amortization of:
Prior service (benefit) cost	(0.6)		(0.6)	(1.9)	(1.8)
Actuarial loss	0.1		0.0	0.1	0.0
Regulatory asset	0.3		0.3	0.8	0.8
Curtailment cost	0.0		0.0	0.0	0.0
Settlement cost	0.0		0.0	0.0	0.0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	$ 2.0		$ 2.0	$ 5.9	$ 6.0