CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Balance at beginning of year		$ 10,283		$ 9,133	$ 10,283	$ 9,133	$ 8,420
Net income (loss)	$ 588	423	$ (101)	288	1,417	1,119	685
Total other comprehensive income (loss)					(69)	36	33
Return on power program appropriation investment					(6)	(5)	(5)
Balance at end of year	11,625		10,283		11,625	10,283	9,133
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		9,404		8,282	9,404	8,282	7,594
Net income (loss)					1,425	1,127	693
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(6)	(5)	(5)
Balance at end of year	10,823		9,404		10,823	9,404	8,282
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		564		572	564	572	580
Net income (loss)					(8)	(8)	(8)
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Balance at end of year	556		564		556	564	572
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		$ 57		$ 21	57	21	(12)
Net income (loss)					0	0	0
Total other comprehensive income (loss)					(69)	36	33
Return on power program appropriation investment					0	0	0
Balance at end of year	$ (12)		$ 57		$ (12)	$ 57	$ 21