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Income Taxes - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Income Taxes [Line Items]          
Effective tax rate 19.86% 39.12% 34.19% 39.10%  
Statutory tax rate     35.00%    
Effective tax rate decrease due to federal research and development credits 19.26% 19.26%      
Income tax examination period     1 year    
Uncertain tax positions $ 7.7   $ 7.7   $ 0.0
Taxes accrued $ 65.0   $ 65.0   13.2
Tampa Electric Company [Member]          
Income Taxes [Line Items]          
Effective tax rate 32.53% 37.73% 34.59% 37.15%  
Effective tax rate decrease due to federal research and development credits 5.20% 5.20%      
Statutes of limitations     3 years    
Income tax examination period     1 year    
Uncertain tax positions $ 6.5   $ 6.5   0.0
Taxes accrued 74.5   74.5   $ 13.2
Tampa Electric Company [Member] | Emera Inc. [Member]          
Income Taxes [Line Items]          
Taxes accrued $ 14.2   $ 14.2    
Minimum [Member]          
Income Taxes [Line Items]          
Statutes of limitations     3 years    
Maximum [Member]          
Income Taxes [Line Items]          
Statutes of limitations     4 years