Fair Value (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ (41)	$ 37	$ (13)	$ 44
Transfers into Level 3	0	1	0	29
Transfer out of Level 3	0	0	(2)	(2)
Total gains: [Abstract]
Included in earnings	15	52	15	45
Recorded in regulatory assets/liabilities	3	4	4	5
Purchases issuances and settlements [Abstract]
Purchases			1	0
Settlements	7	(37)	(21)	(64)
Ending Balance	(16)	57	(16)	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	31	33	20	40
Natural Gas [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	(34)	6	(38)	6
Transfers into Level 3	0	1	0	1
Transfer out of Level 3	0	0	(2)	(2)
Total gains: [Abstract]
Included in earnings	(2)	(1)	(10)	6
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			0	0
Settlements	6	(4)	20	(9)
Ending Balance	(30)	2	(30)	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	(1)	(2)	(6)	4
Electricity [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	(9)	25	23	32
Transfers into Level 3	0	0	0	28
Transfer out of Level 3	0	0	0	0
Total gains: [Abstract]
Included in earnings	18	56	26	41
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			1	0
Settlements	3	(31)	(38)	(51)
Ending Balance	12	50	12	50
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	38	27	38
Other [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	2	6	2	6
Transfers into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Total gains: [Abstract]
Included in earnings	(1)	(3)	(1)	(2)
Recorded in regulatory assets/liabilities	3	4	4	5
Purchases issuances and settlements [Abstract]
Purchases			0	0
Settlements	(2)	(2)	(3)	(4)
Ending Balance	2	5	2	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)	$ (3)	$ (1)	$ (2)