Employee Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2016	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Reclassification of AOCI to net income as part of periodic benefit expense		$ 0.8	$ 0.8
Reclassification of regulatory assets to net income as part of periodic benefit expense		$ 2.2	2.2
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		$ 2.0	1.9
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Long-term EROA		7.00%
Discount rate		4.685%
Employer contributions		$ 4.7	14.9
Net pension expense		$ 4.6	4.4
Pension Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		4.685%
Employer contributions		$ 3.9	11.0
Net pension expense		2.9	2.6
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]
Net pension expense		2.0	2.0
Other Postretirement Benefits [Member] | Tampa Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.667%
Net pension expense		$ 1.5	$ 1.4