Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Mark-to-Market Values of TVA Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	$ 2,250
Other Credit Derivatives [Member] | Other long-term liabilities
Derivatives, Fair Value
Fair value	49	$ 12
Other Credit Derivatives [Member] | Other current assets
Derivatives, Fair Value
Fair value	1,927	1,676
200 million Sterling currency swap
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	209
Fair value	(78)	(90)
200 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	0	(84)
200 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(78)	(6)
250 million Sterling currency swap
Derivatives, Fair Value
Fair value	(63)	(61)
250 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(58)	(56)
250 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(5)	(5)
150 million Sterling currency swap
Derivatives, Fair Value
Fair value	(68)	(57)
150 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(65)	(53)
150 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(3)	(4)
$1.0 billion notional interest rate swap
Derivatives, Fair Value
Fair value	(1,449)	(1,261)
$1.0 billion notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(1,369)	(1,199)
$1.0 billion notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(80)	(62)
$476 million notional interest rate swap
Derivatives, Fair Value
Fair value	(588)	(498)
$476 million notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(556)	(474)
$476 million notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(32)	(24)
$42 million notional interest rate swap
Derivatives, Fair Value
Fair value	(4)	(5)
$42 million notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(2)	(3)
$42 million notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(2)	(2)
Commodity contract derivatives
Derivatives, Fair Value
Fair value	46	(41)
Commodity contract derivatives | Other long-term assets
Derivatives, Fair Value
Fair value	26	0
Commodity contract derivatives | Other long-term liabilities
Derivatives, Fair Value
Fair value	0	(16)
Commodity contract derivatives | Other current assets
Derivatives, Fair Value
Fair value	23	12
Commodity contract derivatives | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	$ (3)	$ (37)