Investments - Interest and Investment Income (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | $ 131 | $ 126 | $ 121 |
| Cash and Cash Equivalents | |||
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | 3 | 0 | 0 |
| Debt Securities | |||
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | 61 | 76 | 82 |
| Equity Securities | |||
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | 28 | 28 | 24 |
| Short-term Investments | |||
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | 4 | 2 | 0 |
| Other Investments | |||
| Schedule of Investment Income, Reported Amounts, by Category | |||
| Interest and investment income | $ 35 | $ 20 | $ 15 |