Summary of Significant Accounting Policies (Details) (USD $) In Millions | 3 Months Ended | 9 Months Ended | ||
---|---|---|---|---|
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | |
Accounts Receivable (Numeric) [Abstract] | ||||
Purchases of accounts receivable | $ 219 | $ 203 | $ 671 | $ 428 |