Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.5	$ 4.1
Interest cost	7.4	8.2
Expected return on assets	(10.8)	(10.3)
Amortization of:
Prior service (benefit) cost	(0.1)	(0.1)
Actuarial loss	3.4	3.2
Regulatory asset	0	0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	4.4	5.1
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.6
Interest cost	2.0	2.6
Expected return on assets	(0.3)	0
Amortization of:
Prior service (benefit) cost	(0.6)	0
Actuarial loss	0	0
Regulatory asset	0.3	0
Net pension expense recognized in the TECO Energy Consolidated Condensed Statements of Income	$ 2.0	$ 3.2