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Proprietary Capital Proprietary Capital (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Appropriation Investment
Amount of appropriation investment that was repaid	$ 1,000								$ 1,000
Remaining appropriation investment	258								258
Balance at beginning of year				$ 9,133				$ 8,420	9,133	$ 8,420	$ 7,203
Net income (loss)	(101)	$ 470	$ 462	288	$ 139	$ 233	$ 211	102	1,119	685	1,233
Return on power program appropriation investment									(5)	(5)	(6)
Balance at end of year	10,283				9,133				10,283	9,133	$ 8,420
Net proprietary capital at September 30	$ 10,283				$ 9,133				$ 10,283	$ 9,133
Computed average interest rate payable	2.09%				2.00%				2.09%	2.00%	2.04%
Nonpower Programs Appropriation Investment
Appropriation Investment
Balance at beginning of year				4,351					$ 4,351
Balance at end of year	$ 4,351				$ 4,351				4,351	$ 4,351
Power Program Appropriation Investment
Appropriation Investment
Balance at beginning of year				258				258	258	258	$ 258
Net income (loss)									0	0	0
Return on power program appropriation investment									0	0	0
Balance at end of year	258				258				258	258	258
Power Program Retained Earnings
Appropriation Investment
Balance at beginning of year				8,282				7,594	8,282	7,594	6,357
Net income (loss)									1,127	693	1,243
Return on power program appropriation investment									(5)	(5)	(6)
Balance at end of year	9,404				8,282				9,404	8,282	7,594
Net proprietary capital at September 30	9,662				8,540				9,662	8,540
Nonpower Programs Appropriation Investment, Net
Appropriation Investment
Balance at beginning of year				572				580	572	580	590
Net income (loss)									(8)	(8)	(10)
Return on power program appropriation investment									0	0	0
Balance at end of year	564				572				564	572	580
Nonpower Programs Retained Earnings
Appropriation Investment
Balance at beginning of year				$ (3,779)				$ (3,771)	(3,779)	(3,771)
Return on power program appropriation investment									0	0
Balance at end of year	(3,787)				(3,779)				(3,787)	(3,779)	(3,771)
Net proprietary capital at September 30	$ 564				$ 572				564	572
Affiliated Entity
Appropriation Investment
Return on power program appropriation investment									$ (5)	$ (5)	$ (6)

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- Definition

The amount of the appropriation investment from Congress that must be repaid.

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Appropriation Investment [Line Items]

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The computed average interest rate payable by the U.S. Treasury on its total marketable public obligations.

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Total proprietary capital.

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- Definition

The remaining appropriation investment if no additional appropriations from Congress are received.

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This element represents disclosure of the cash payments from net power proceeds paid annually to the U.S. government as a return on the government's appropriation investment in the reporting entity.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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