CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Balance at beginning of year		$ 7,203		$ 6,104	$ 7,203	$ 6,104	$ 5,647
Net income (loss)	$ 661	(37)	$ 502	81	1,233	1,111	469
Total other comprehensive income (loss)					(10)	(7)	2
Return on power program appropriation investment					(6)	(5)	(4)
Return of power program appropriation investment							(10)
Balance at end of year	8,420		7,203		8,420	7,203	6,104
Power Program Appropriation Investment
Balance at beginning of year		258		258	258	258	268
Net income (loss)					0	0	0
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Return of power program appropriation investment					0	0	(10)
Balance at end of year	258		258		258	258	258
Power Program Retained Earnings
Balance at beginning of year		6,357		5,240	6,357	5,240	4,767
Net income (loss)					1,243	1,122	477
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(6)	(5)	(4)
Return of power program appropriation investment							0
Balance at end of year	7,594		6,357		7,594	6,357	5,240
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		590		601	590	601	609
Net income (loss)					(10)	(11)	(8)
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Return of power program appropriation investment							0
Balance at end of year	580		590		580	590	601
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		$ (2)		$ 5	(2)	5	3
Net income (loss)					0	0	0
Total other comprehensive income (loss)					(10)	(7)	2
Return on power program appropriation investment					0	0	0
Return of power program appropriation investment							0
Balance at end of year	$ (12)		$ (2)		$ (12)	$ (2)	$ 5