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Short Term Debt (Narrative) (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 19, 2019
USD ($)
Dec. 16, 2019
CAD ($)
Mar. 07, 2019
USD ($)
Dec. 31, 2019
CAD ($)
Feb. 06, 2020
USD ($)
Dec. 20, 2019
USD ($)
Dec. 31, 2018
CAD ($)
Line of Credit Facility [Line Items]              
Weighted average interest rate       2.54%     3.34%
Line of Credit Facility, Maximum Borrowing Capacity       $ 2,463     $ 2,065
Revolving credit facility | TECO Energy/TECO Finance              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date       Mar. 22, 2022      
Line of Credit Facility, Maximum Borrowing Capacity       $ 520     546
Revolving credit facility | Tampa Electric Company              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date Feb. 04, 2021     Mar. 22, 2022      
Line of Credit Facility, Maximum Borrowing Capacity $ 325     $ 520 $ 300 $ 400 443
Line of Credit Facility, Increase (Decrease), Net $ 75            
Revolving credit facility | NMGC              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date       Mar. 22, 2022      
Line of Credit Facility, Maximum Borrowing Capacity       $ 162     171
Revolving credit facility | GBPC              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Maximum Borrowing Capacity       $ 17     18
Accounts receivable revolving credit facility | Tampa Electric Company              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date       Dec. 31, 2021      
Line of Credit Facility, Maximum Borrowing Capacity       $ 195     205
Term bank credit facility | TECO Energy/TECO Finance              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date       Dec. 31, 2020      
Line of Credit Facility, Maximum Borrowing Capacity       $ 649     $ 682
Non-revolving term credit facility              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date   Dec. 15, 2020          
Line of Credit Facility, Maximum Borrowing Capacity   $ 400          
Non-revolving term credit facility | TECO Energy/TECO Finance              
Line of Credit Facility [Line Items]              
Line of Credit Facility, Expiration Date     Mar. 05, 2020        
Line of Credit Facility, Maximum Borrowing Capacity     $ 500