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Valuation of Debt and Equity Investments and Certain Liabilities - Additional Information (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
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 $ 1,380,000,000 $ 1,270,000,000 2,830,000,000 $ 2,385,000,000  
Fair Value, Measurements, Recurring          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Total assets fair value 5,210,000,000   5,210,000,000   $ 4,446,000,000
Total liabilities fair value 263,000,000   263,000,000   255,000,000
Fair Value, Measurements, Recurring | Level 3          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Total assets fair value 0   0   0
Total liabilities fair value $ 0   $ 0   $ 0