Fair Value (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ (16)	$ 57	$ (13)	$ 44
Transfers into Level 3	0	0	0	29
Transfer out of Level 3	1	0	2	(2)
Total gains (losses): [Abstract]
Included in earnings	25	(30)	43	15
Recorded in regulatory assets/liabilities	3	7	7	12
Purchases issuances and settlements [Abstract]
Purchases			(1)	1
Settlements	(15)	(48)	(38)	(113)
Ending Balance	(4)	(14)	(4)	(14)
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	(37)	22	4
Natural Gas Commodity Contract [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	(30)	2	(38)	6
Transfers into Level 3	0	0	0	1
Transfer out of Level 3	0	0	2	(2)
Total gains (losses): [Abstract]
Included in earnings	8	(43)	(1)	(38)
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			0	0
Settlements	8	(3)	27	(11)
Ending Balance	(14)	(44)	(14)	(44)
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)	(43)	(14)	(39)
Electricity [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	12	50	23	32
Transfers into Level 3	0	0	0	28
Transfer out of Level 3	1	0	0	0
Total gains (losses): [Abstract]
Included in earnings	17	12	44	53
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			(1)	1
Settlements	(23)	(38)	(61)	(90)
Ending Balance	5	24	5	24
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	5	36	43
Other Commodity Contract [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	2	5	2	6
Transfers into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Total gains (losses): [Abstract]
Included in earnings	0	1	0	0
Recorded in regulatory assets/liabilities	3	7	7	12
Purchases issuances and settlements [Abstract]
Purchases			0	0
Settlements	0	(7)	(4)	(12)
Ending Balance	5	6	5	6
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	$ 1	$ 0	$ 0