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Summary of Significant Accounting Policies (Schedule of Excise Taxes) (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        
Excise tax expense $ 44 $ 44 $ 95 $ 90