Employee Postretirement Benefits - Schedule of Amounts Recognized in Net Periodic Benefit Cost, OCI, and Regulatory Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Amortization, settlement, or curtailment of:
Net periodic benefit cost	$ 13.3	$ 13.5	$ 14.8
Other Postretirement Benefits [Member]
Amortization, settlement, or curtailment of:
Net periodic benefit cost	6.4	5.7	10.4
TECO Energy [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	18.8	20.9	18.3
Interest cost	30.8	30.3	32.0
Expected return on plan assets	(45.8)	(43.3)	(41.8)
Amortization, settlement, or curtailment of:
Actuarial loss	16.4	15.1	13.5
Prior service (benefit) cost	0.3	(0.2)	(0.4)
Curtailment loss (gain)	1.3	0.0	3.9
Special termination benefit	0.0	0.0	0.2
Settlement loss	0.6	0.0	0.0
Net periodic benefit cost	22.4	22.8	25.7
New prior service cost	1.3	0.0	0.0
Net loss (gain) arising during the year	46.8	74.5	44.1
Amounts recognized as component of net periodic benefit cost:
Amortization or curtailment recognition of prior service (benefit) cost	(0.3)	0.2	0.4
Amortization or settlement of actuarial gain (loss)	(17.1)	(15.1)	(13.5)
Total recognized in OCI and regulatory assets	30.7	59.6	31.0
Total recognized in net periodic benefit cost, OCI and regulatory assets	53.1	82.4	56.7
TECO Energy [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8	1.9	2.4
Interest cost	7.4	7.0	10.4
Expected return on plan assets	0.0	0.0	0.0
Amortization, settlement, or curtailment of:
Actuarial loss	0.2	0.0	0.2
Prior service (benefit) cost	(2.4)	(2.4)	(0.2)
Curtailment loss (gain)	0.0	0.0	(0.2)
Special termination benefit	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic benefit cost	7.0	6.5	12.6
New prior service cost	0.0	0.0	(23.2)
Net loss (gain) arising during the year	5.0	0.4	(10.1)
Amounts recognized as component of net periodic benefit cost:
Amortization or curtailment recognition of prior service (benefit) cost	2.4	2.5	0.3
Amortization or settlement of actuarial gain (loss)	(0.2)	0.0	(0.2)
Total recognized in OCI and regulatory assets	7.2	2.9	(33.2)
Total recognized in net periodic benefit cost, OCI and regulatory assets	$ 14.2	$ 9.4	$ (20.6)