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Employee Postretirement Benefits - Schedule of One-percentage-point Change in Assumed Health Care Cost (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on periodic cost - Increase $ 0.4
Effect on periodic cost - Decrease (0.3)
Tampa Electric Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Effect on periodic cost - Increase 0.2
Effect on periodic cost - Decrease (0.2)
Assumed One-percentage-point Change [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Effect on postretirement benefit obligation - Increase 9.0
Effect on postretirement benefit obligation - Decrease (7.7)
Assumed One-percentage-point Change [Member] | Tampa Electric Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Effect on postretirement benefit obligation - Increase 6.1
Effect on postretirement benefit obligation - Decrease $ (5.2)