Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.6	$ 4.5	$ 12.9	$ 13.6
Interest cost on projected benefit obligations	8.0	7.3	24.4	21.7
Expected return on assets	(10.5)	(9.6)	(31.2)	(28.8)
Amortization of:
Prior service (benefit) cost	(0.1)	(0.1)	(0.3)	(0.3)
Actuarial loss	3.3	5.1	10.0	15.4
Regulatory asset	0	0	0	0
Settlement cost	0	1.0	0	1.0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	5.3	8.2	15.8	22.6
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.7	1.8	1.9
Interest cost on projected benefit obligations	2.7	2.3	7.9	7.0
Expected return on assets	(0.1)	0	(0.1)	0
Amortization of:
Prior service (benefit) cost	0	(0.1)	(0.1)	(0.3)
Actuarial loss	0	0.2	0.1	0.7
Regulatory asset	0.1	0	0.1	0
Settlement cost	0	0	0	0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	$ 3.3	$ 3.1	$ 9.7	$ 9.3