Summary of Significant Accounting Policies (Schedule of Amortization Expense For Intangible Assets) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Summary of Significant Accounting Policies
Amortization expense based on usage of emission allowances	$ 1	[1]	$ 7	[1]	$ 2	[1]	$ 10	[1]

[1] Includes allowances consumed that were recorded through purchase accounting.