Fair Value (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ 44	$ 59
Transfers into Level 3	27	2
Transfer out of Level 3	(2)	(22)
Total gains: [Abstract]
Included in earnings	(7)	(17)
Recorded in regulatory assets/liabilities	1	(1)
Purchases issuances and settlements [Abstract]
Settlements	(26)	(5)
Ending Balance	37	16
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	8	(7)
Natural Gas [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	6	1
Transfers into Level 3	0	0
Transfer out of Level 3	(2)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3		3
Total gains: [Abstract]
Included in earnings	6	(4)
Recorded in regulatory assets/liabilities	0	0
Purchases issuances and settlements [Abstract]
Settlements	(4)	3
Ending Balance	6	3
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	6	(1)
Electricity [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	32	54
Transfers into Level 3	27	2
Transfer out of Level 3	0	(25)
Total gains: [Abstract]
Included in earnings	(14)	(15)
Recorded in regulatory assets/liabilities	0	0
Purchases issuances and settlements [Abstract]
Settlements	(20)	(8)
Ending Balance	25	8
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	1	(8)
Other [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	6	4
Transfers into Level 3	0	0
Transfer out of Level 3	0	0
Total gains: [Abstract]
Included in earnings	1	2
Recorded in regulatory assets/liabilities	1	(1)
Purchases issuances and settlements [Abstract]
Settlements	(2)	0
Ending Balance	6	5
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	$ 1	$ 2