Investments (Details) - USD ($) $ in Millions |
Sep. 30, 2021 |
Dec. 31, 2020 |
|---|---|---|
| Debt Securities, Available-for-sale [Line Items] | ||
| Amortized cost | $ 12,275 | $ 9,844 |
| Gross unrealized gains | 0 | 1 |
| Gross unrealized losses | 0 | 0 |
| Fair values | 12,275 | 9,845 |
| U.S. Treasury notes [Member] | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Amortized cost | 51 | 129 |
| Gross unrealized gains | 0 | 1 |
| Gross unrealized losses | 0 | 0 |
| Fair values | 51 | 130 |
| U.S. Treasury bills [Member] | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Amortized cost | 3,900 | 4,948 |
| Gross unrealized gains | 0 | 0 |
| Gross unrealized losses | 0 | 0 |
| Fair values | 3,900 | 4,948 |
| Money market mutual funds [Member] | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Amortized cost | 8,323 | 4,765 |
| Gross unrealized gains | 0 | 0 |
| Gross unrealized losses | 0 | 0 |
| Fair values | 8,323 | 4,765 |
| Other short-term interest-bearing securities [Member] | ||
| Debt Securities, Available-for-sale [Line Items] | ||
| Amortized cost | 1 | 2 |
| Gross unrealized gains | 0 | 0 |
| Gross unrealized losses | 0 | 0 |
| Fair values | $ 1 | $ 2 |