Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6.7	$ 4.6	$ 17.6	$ 12.9
Interest cost	6.5	8.0	22.6	24.4
Expected return on assets	(9.1)	(10.5)	(32.4)	(31.2)
Amortization of:
Prior service (benefit) cost	(0.1)	(0.1)	(0.2)	(0.3)
Actuarial loss	3.2	3.3	11.4	10.0
Regulatory asset	0.0	0.0	0.0	0.0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	7.2	5.3	19.0	15.8
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.6	1.7	1.8
Interest cost	2.0	2.7	6.1	7.9
Expected return on assets	(0.3)	(0.1)	(0.8)	(0.1)
Amortization of:
Prior service (benefit) cost	(0.6)	0.0	(1.8)	(0.1)
Actuarial loss	0.0	0.0	0.0	0.1
Regulatory asset	0.3	0.1	0.8	0.1
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	$ 2.0	$ 3.3	$ 6.0	$ 9.7