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 | ||