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Valuation of Debt and Equity Investments and Certain Liabilities - Additional Information (Details) - USD ($)
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Credit losses related to available-for-sale investments $ 0 $ 0  
Proceeds from sales, redemptions and maturities of short-term available-for-sale securities 2,455,000,000 $ 2,491,000,000  
Fair Value, Measurements, Recurring      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Total assets fair value 2,666,000,000   $ 3,511,000,000
Total liabilities fair value 191,000,000   202,000,000
Fair Value, Measurements, Recurring | Level 3      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Total assets fair value 0   0
Total liabilities fair value $ 0   $ 0