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Risk Management Activities and Derivative Transactions Currency Swaps Outstanding (Details)
$ in Millions
3 Months Ended
Dec. 31, 2017
USD ($)
Bond_issues
Derivative  
Number of British pound sterling denominated bond transactions 3
Number of currency swaps outstanding 3
Associated TVA bond issues currency exposure | $ $ 600
Minimum  
Derivative  
Expiration date range of swaps 2021
Maximum  
Derivative  
Expiration date range of swaps 2043