CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Balance at beginning of year		$ 6,104		$ 5,647	$ 6,104	$ 5,647	$ 5,326
Net income (loss)	$ 502	81	$ 322	(67)	1,111	469	271
Total other comprehensive income (loss)					(7)	2	77
Return on power program appropriation investment					(5)	(4)	(7)
Return of power program appropriation investment						(10)	(20)
Balance at end of year	7,203		6,104		7,203	6,104	5,647
Power Program Appropriation Investment
Balance at beginning of year		258		268	258	268	288
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	(10)	(20)
Balance at end of year	258		258		258	258	268
Power Program Retained Earnings
Balance at beginning of year		5,240		4,767	5,240	4,767	4,492
Net income (loss)					1,122	477	282
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(5)	(4)	(7)
Return of power program appropriation investment						0	0
Balance at end of year	6,357		5,240		6,357	5,240	4,767
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		601		609	601	609	620
Net income (loss)					(11)	(8)	(11)
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
Balance at end of year	590		601		590	601	609
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		$ 5		$ 3	5	3	(74)
Net income (loss)					0	0	0
Total other comprehensive income (loss)					(7)	2	77
Return on power program appropriation investment					0	0	0
Return of power program appropriation investment						0	0
Balance at end of year	$ (2)		$ 5		$ (2)	$ 5	$ 3