Investments and Fair Value Measurement - Fair Value of Unobservable Inputs, Assets (Details) - USD ($) $ in Millions |
6 Months Ended | 9 Months Ended |
|---|---|---|
Jun. 30, 2019 |
Sep. 30, 2019 |
|
| Debt Securities | ||
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||
| Balance as of December 31, 2018 | $ 2,370 | $ 2,370 |
| Total net gains (losses) | ||
| Included in earnings | (8) | |
| Included in other comprehensive income (loss) | 0 | |
| Purchases | 4 | |
| Transfers | 0 | |
| Balance as of September 30, 2019 | 2,366 | |
| Non-marketable equity securities | ||
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] | ||
| Balance as of December 31, 2018 | 0 | 0 |
| Total net gains (losses) | ||
| Included in earnings | 22 | (12) |
| Included in other comprehensive income (loss) | 0 | |
| Purchases | 11 | |
| Transfers | 88 | |
| Balance as of September 30, 2019 | $ 87 | |
| Warrants vested during period | $ 35 |