Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Amortized Cost | ||
| Due in one year or less | $ 1,576 | |
| Due after one year through five years | 959 | |
| Due after five years through ten years | 1 | |
| Amortized Cost | 2,536 | $ 2,700 |
| Estimated Fair Value | ||
| Due in one year or less | 1,573 | |
| Due after one year through five years | 947 | |
| Due after five years through ten years | 1 | |
| Estimated Fair Value | $ 2,521 |