Fair Value (Details 1) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ 16	$ 96	$ 59	$ 24
Transfer into Level 3	59		73
Transfer out of Level 3	(2)	138	(24)	86
Total gains or (losses):
Included in earnings	11	(51)	(18)	85
Recorded in regulatory assets/liabilities	4	4	3	3
Purchases, issuances, sales and settlements:
Purchases	1		1
Settlements	(24)		(29)
Purchases, issuances and settlements		(26)		(37)
Ending balance	65	161	65	161
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period	4	(74)	(16)	45
Natural Gas [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	3	5	1	2
Transfer into Level 3	(3)		0
Transfer out of Level 3	0		1	3
Total gains or (losses):
Included in earnings	3		(2)	2
Recorded in regulatory assets/liabilities	0		0
Purchases, issuances, sales and settlements:
Purchases	0		0
Settlements	(2)		1
Purchases, issuances and settlements		(3)		(5)
Ending balance	1	2	1	2
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period	2	(3)	(1)	(4)
Electricity [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	8	89	54	19
Transfer into Level 3	62		73
Transfer out of Level 3	(2)	138	(25)	83
Total gains or (losses):
Included in earnings	8	(51)	(18)	83
Recorded in regulatory assets/liabilities	0		0
Purchases, issuances, sales and settlements:
Purchases	1		1
Settlements	(20)		(28)
Purchases, issuances and settlements		(21)		(30)
Ending balance	57	155	57	155
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period	2	(71)	(17)	49
Other [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	5	2	4	3
Transfer into Level 3	0		0
Transfer out of Level 3	0		0
Total gains or (losses):
Included in earnings	0		2
Recorded in regulatory assets/liabilities	4	4	3	3
Purchases, issuances, sales and settlements:
Purchases	0		0
Settlements	(2)		(2)
Purchases, issuances and settlements		(2)		(2)
Ending balance	7	4	7	4
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period	$ 0		$ 2