Fair Value (Reconciliation of Level 3 Assets and Liabilities at Fair Value on a Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	$ (40)	$ (16)	$ (36)	$ (13)
Transfers into Level 3	1	0	0	0
Transfer out of Level 3	(4)	(1)	2	(2)
Total gains (losses): [Abstract]
Included in earnings	10	25	(21)	43
Recorded in regulatory assets/liabilities	(3)	3	9	7
Purchases issuances and settlements [Abstract]
Purchases			1	(1)
Issuances			(2)	0
Settlements	(6)	(15)	5	(38)
Ending Balance	(42)	(4)	(42)	(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	6	0	(30)	22
Natural Gas Commodity Contract [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	(20)	(30)	(52)	(38)
Transfers into Level 3	1	0	0	0
Transfer out of Level 3	(4)	0	2	(2)
Total gains (losses): [Abstract]
Included in earnings	(2)	8	(45)	(1)
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			0	0
Issuances			0	0
Settlements	(3)	8	67	27
Ending Balance	(28)	(14)	(28)	(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	(7)	(8)	(23)	(14)
Electricity Commodity Contract [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	(27)	12	13	23
Transfers into Level 3	0	0	0	0
Transfer out of Level 3	0	(1)	0	0
Total gains (losses): [Abstract]
Included in earnings	13	17	26	44
Recorded in regulatory assets/liabilities	0	0	0	0
Purchases issuances and settlements [Abstract]
Purchases			1	(1)
Issuances			(2)	0
Settlements	(3)	(23)	(55)	(61)
Ending Balance	(17)	5	(17)	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	14	8	(6)	36
Other Commodity Contract [Member]
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis
Beginning Balance	7	2	3	2
Transfers into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Total gains (losses): [Abstract]
Included in earnings	(1)	0	(2)	0
Recorded in regulatory assets/liabilities	(3)	3	9	7
Purchases issuances and settlements [Abstract]
Purchases			0	0
Issuances			0	0
Settlements	0	0	(7)	(4)
Ending Balance	3	5	3	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)	$ 0	$ (1)	$ 0