Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Remaining Recovery Period of Regulatory Assets, Price Risk Management	2 years
Remaining Recovery Period of Regulatory Asset, Deferred Broker Settlements	1 year
Remaining Recovery Period of Regulatory Asset, Debt Reacquisition Costs	7 years
Price risk management	$ 4	$ 19
Deferred income taxes	20	22
Remaining refund period of power cost adjustment mechanism	1 year
Asset retirement obligations	94	87
Current Regulatory Liabilities [Member]
Regulatory Assets [Line Items]
Asset retirement removal costs	0	0
Asset retirement obligations	0	0
Power Cost Adjustment Mechanism	6	0
Other	6	6
Total regulatory liabilities	12	6
Noncurrent Regulatory Liabilities [Member]
Regulatory Assets [Line Items]
Asset retirement removal costs	692	637
Asset retirement obligations	39	36
Power Cost Adjustment Mechanism	0	10
Other	34	37
Total regulatory liabilities	765	720
Regulatory Assets, Current [Member]
Regulatory Assets [Line Items]
Price risk management	123	194
Pension and other postretirement plans	0	0
Deferred income taxes	0	0
Deferred broker settlements	20	11
Debt reacquisition costs	0	0
Deferred capital project	0	0
Other	1	11
Total regulatory assets	144	216
Noncurrent Regulatory Assets [Member]
Regulatory Assets [Line Items]
Price risk management	71	172
Pension and other postretirement plans	321	295
Deferred income taxes	80	87
Deferred broker settlements	1	0
Debt reacquisition costs	22	28
Deferred capital project	16	0
Other	13	12
Total regulatory assets	$ 524	$ 594