Fair Value (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ 37	$ 16	$ 44	$ 59
Transfers into Level 3	1	59	29	73
Transfer out of Level 3	0	(2)	(2)	(24)
Total gains: [Abstract]
Included in earnings	52	11	45	(18)
Recorded in regulatory assets/liabilities	4	4	5	3
Purchases issuances and settlements [Abstract]
Purchases	0	1	0	1
Settlements	(37)	(24)	(64)	(29)
Ending Balance	57	65	57	65
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	33	4	40	(16)
Natural Gas [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	6	3	6	1
Transfers into Level 3	1	(3)	1	0
Transfer out of Level 3	0	0	(2)	1
Total gains: [Abstract]
Included in earnings	(1)	3	6	(2)
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases	0	0	0	0
Settlements	(4)	(2)	(9)	1
Ending Balance	2	1	2	1
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)	2	4	(1)
Electricity [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	25	8	32	54
Transfers into Level 3	0	62	28	73
Transfer out of Level 3	0	(2)	0	(25)
Total gains: [Abstract]
Included in earnings	56	8	41	(18)
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases	0	1	0	1
Settlements	(31)	(20)	(51)	(28)
Ending Balance	50	57	50	57
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	38	2	38	(17)
Other Credit Derivatives [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	6	5	6	4
Transfers into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Total gains: [Abstract]
Included in earnings	(3)	0	(2)	2
Recorded in regulatory assets/liabilities	4	4	5	3
Purchases issuances and settlements [Abstract]
Purchases	0	0	0	0
Settlements	(2)	(2)	(4)	(2)
Ending Balance	5	7	5	7
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	$ (3)	$ 0	$ (2)	$ 2