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, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20.9	$ 18.3	$ 18.2
Interest cost	30.3	32.0	28.9
Expected return on plan assets	(43.3)	(41.8)	(38.4)
Amortization of:
Actuarial loss	15.1	13.5	20.5
Prior service (benefit) cost	(0.2)	(0.4)	(0.4)
Curtailment loss (gain)	0.0	3.9	0.0
Special termination benefit	0.0	0.2	0.0
Settlement loss	0.0	0.0	1.0
Net periodic benefit cost	22.8	25.7	29.8
New prior service cost	0.0	0.0	0.0
Net loss (gain) arising during the year	74.5	44.1	(75.7)
Unrecognized costs in regulated asset acquired in business combination	0.0	0.0	0.0
Amounts recognized as component of net periodic benefit cost:
Amortization of actuarial gain (loss)	(15.1)	(13.5)	(21.5)
Amortization of prior service (benefit) cost	0.2	0.4	0.4
Total recognized in OCI and regulatory assets	59.6	31.0	(96.8)
Total recognized in net periodic benefit cost, OCI and regulatory assets	82.4	56.7	(67.0)
Pension Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20.9	18.3	18.2
Interest cost	30.3	32.0	28.9
Expected return on plan assets	(43.3)	(41.8)	(38.4)
Amortization of:
Actuarial loss	15.1	13.5	20.5
Prior service (benefit) cost	(0.2)	(0.4)	(0.4)
Curtailment loss (gain)	0.0	3.9	0.0
Special termination benefit	0.0	0.2	0.0
Settlement loss	0.0	0.0	1.0
Net periodic benefit cost	22.8	25.7	29.8
New prior service cost	0.0	0.0	0.0
Net loss (gain) arising during the year	74.5	44.1	(75.7)
Unrecognized costs in regulated asset acquired in business combination	0.0	0.0	0.0
Amounts recognized as component of net periodic benefit cost:
Amortization of actuarial gain (loss)	(15.1)	(13.5)	(21.5)
Amortization of prior service (benefit) cost	0.2	0.4	0.4
Total recognized in OCI and regulatory assets	59.6	31.0	(96.8)
Total recognized in net periodic benefit cost, OCI and regulatory assets	82.4	56.7	(67.0)
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.2	2.5	2.5
Interest cost	8.2	10.8	9.3
Expected return on plan assets	(1.1)	(0.3)	0.0
Amortization of:
Actuarial loss	0.0	0.2	1.0
Prior service (benefit) cost	(2.4)	(0.2)	(0.4)
Curtailment loss (gain)	0.0	(0.2)	0.0
Special termination benefit	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic benefit cost	6.9	12.8	12.4
New prior service cost	(3.7)	(23.6)	0.0
Net loss (gain) arising during the year	1.3	(9.9)	(15.6)
Unrecognized costs in regulated asset acquired in business combination	0.0	6.4	0.0
Amounts recognized as component of net periodic benefit cost:
Amortization of actuarial gain (loss)	0.0	(0.2)	(1.0)
Amortization of prior service (benefit) cost	2.4	0.2	0.3
Total recognized in OCI and regulatory assets	0.0	(27.1)	(16.3)
Total recognized in net periodic benefit cost, OCI and regulatory assets	6.9	(14.3)	(3.9)
Other Postretirement Benefits [Member] | TECO Energy [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.2	2.5	2.5
Interest cost	8.2	10.8	9.3
Expected return on plan assets	(1.1)	(0.3)	0.0
Amortization of:
Actuarial loss	0.0	0.2	1.0
Prior service (benefit) cost	(2.4)	(0.2)	(0.4)
Curtailment loss (gain)	0.0	(0.2)	0.0
Special termination benefit	0.0	0.0	0.0
Settlement loss	0.0	0.0	0.0
Net periodic benefit cost	6.9	12.8	12.4
New prior service cost	(3.7)	(23.6)	0.0
Net loss (gain) arising during the year	1.3	(9.9)	(15.6)
Unrecognized costs in regulated asset acquired in business combination	0.0	6.4	0.0
Amounts recognized as component of net periodic benefit cost:
Amortization of actuarial gain (loss)	0.0	(0.2)	(1.0)
Amortization of prior service (benefit) cost	2.4	0.2	0.3
Total recognized in OCI and regulatory assets	0.0	(27.1)	(16.3)
Total recognized in net periodic benefit cost, OCI and regulatory assets	$ 6.9	$ (14.3)	$ (3.9)