CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Balance at beginning of year		$ 9,133		$ 8,420	$ 9,133	$ 8,420	$ 7,203
Net income (loss)	$ (101)	288	$ 139	102	1,119	685	1,233
Total other comprehensive income (loss)					36	33	(10)
Return on power program appropriation investment					(5)	(5)	(6)
Balance at end of year	10,283		9,133		10,283	9,133	8,420
Power Program Appropriation Investment
Balance at beginning of year		258		258	258	258	258
Net income (loss)					0	0	0
Total other comprehensive 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
Balance at beginning of year		8,282		7,594	8,282	7,594	6,357
Net income (loss)					1,127	693	1,243
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(5)	(5)	(6)
Balance at end of year	9,404		8,282		9,404	8,282	7,594
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		572		580	572	580	590
Net income (loss)					(8)	(8)	(10)
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Balance at end of year	564		572		564	572	580
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		$ 21		$ (12)	21	(12)	(2)
Net income (loss)					0	0	0
Total other comprehensive income (loss)					36	33	(10)
Return on power program appropriation investment					0	0	0
Balance at end of year	$ 57		$ 21		$ 57	$ 21	$ (12)