Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Jan. 01, 2016	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Reclassification of AOCI to net income as part of periodic benefit expense			$ 0.2	$ 1.4	$ 1.0	$ 2.2
Reclassification of regulatory assets to net income as part of periodic benefit expense			3.1	3.0	$ 5.3	5.2
Curtailment cost			1.3
Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA					7.00%
Reclassification of regulatory assets to net income as part of periodic benefit expense			2.8	2.8	$ 4.8	4.7
Scenario Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement charge related to the SERP		$ 8.0
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA					7.00%
Discount rate					4.685%
Employer contributions					$ 15.6	24.5
Curtailment cost			1.3	0.0	1.3	0.0
Settlement charge related to the SERP			(0.6)	0.0	(0.6)	0.0
Net pension expense			7.1	7.4	$ 11.7	11.8
Pension Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate					4.685%
Employer contributions					$ 12.9	18.5
Net pension expense			3.2	4.2	6.1	6.8
Other Postretirement Benefits Florida-Based Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.667%
Other Postretirement Benefits, NMGC Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.687%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment cost			0.0	0.0	0.0	0.0
Settlement charge related to the SERP			0.0	0.0	0.0	0.0
Net pension expense			1.9	2.0	3.9	4.0
Other Postretirement Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.667%
Net pension expense			$ 1.5	$ 1.5	$ 3.0	$ 2.9