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Valuation of Debt and Equity Investments and Certain Liabilities - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Credit losses related to available-for-sale investments $ 0 $ 0  
Proceeds from short-term investments 900,000,000 $ 615,000,000  
Fair Value, Measurements, Recurring      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Total assets fair value 2,757,000,000   $ 3,177,000,000
Total liabilities fair value 202,000,000   198,000,000
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