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Short Term Debt (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Nov. 02, 2017
Mar. 22, 2017
Mar. 08, 2017
Revolving Credit Facility [Member] | New Mexico Gas Company [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity       $ 125  
Line of Credit Facility, Expiration Date Mar. 22, 2022 Dec. 17, 2018      
Revolving Credit Facility [Member] | TECO Energy Inc. and TECO Finance Inc. [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity       300  
Revolving Credit Facility [Member] | TECO Energy Inc. and TECO Finance Inc. [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Expiration Date Mar. 22, 2022 Dec. 17, 2018      
Letter of Credit [Member] | Tampa Electric Division [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity       50  
Finance Term Credit Facility [Member] | Tampa Electric Division [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity       $ 325  
Line of Credit Facility, Expiration Date Mar. 22, 2022 Dec. 17, 2018      
Finance Term Credit Facility [Member] | Tampa Electric Division [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity     $ 300    
Line of Credit Facility, Expiration Date Nov. 01, 2018        
Finance Term Credit Facility [Member] | TECO Energy Inc. and TECO Finance Inc. [Member]          
Line of Credit Facility [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity         $ 400
Line of Credit Facility, Expiration Date Mar. 08, 2018 Mar. 14, 2017