CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Balance at beginning of year		$ 5,647		$ 5,326	$ 5,647	$ 5,326	$ 5,229
Net income (loss)	322	(67)	474	(245)	469	271	60
Total other comprehensive income (loss)					2	77	64
Return on power program appropriation investment					(4)	(7)	(7)
Return of power program appropriation investment					(10)	(20)	(20)
Balance at end of year	6,104		5,647		6,104	5,647	5,326
Power Program Appropriation Investment
Balance at beginning of year		268		288	268	288	308
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					(10)	(20)	(20)
Balance at end of year	258		268		258	268	288
Power Program Retained Earnings
Balance at beginning of year		4,767		4,492	4,767	4,492	4,429
Net income (loss)					477	282	70
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(4)	(7)	(7)
Return of power program appropriation investment					0	0	0
Balance at end of year	5,240		4,767		5,240	4,767	4,492
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		609		620	609	620	630
Net income (loss)					(8)	(11)	(10)
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	0
Balance at end of year	601		609		601	609	620
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		3		(74)	3	(74)	(138)
Net income (loss)					0	0	0
Total other comprehensive income (loss)					2	77	64
Return on power program appropriation investment					0	0	0
Return of power program appropriation investment					0	0	0
Balance at end of year	$ 5		$ 3		$ 5	$ 3	$ (74)