Investments - Held to Maturity Investments (Details) $ in Millions |
Mar. 31, 2018
USD ($)
|
|---|---|
| Amortized Cost | |
| Due in one year or less | $ 117 |
| Due after one year through five years | 3 |
| Held-to-maturity Securities | 120 |
| Estimated Fair Value | |
| Due in one year or less | 117 |
| Due after one year through five years | 3 |
| Estimated Fair Value, Total | $ 120 |