Fair Value Disclosures (Measured Based on Net Asset Value) (Details) - USD ($) $ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Measured Based on Net Asset Value | ||
| Carrying value | $ 2,573 | $ 2,564 |
| Commitment | 489 | 495 |
| Private Equity Funds | ||
| Measured Based on Net Asset Value | ||
| Carrying value | 1,678 | 1,674 |
| Commitment | 304 | 308 |
| Carrying Value of Nonredeemable Funds by Contractual Maturity | ||
| Less than 5 years | 430 | |
| 5-10 years | 1,054 | |
| Over 10 years | 194 | |
| Total | 1,678 | |
| Real Estate Funds | ||
| Measured Based on Net Asset Value | ||
| Carrying value | 796 | 800 |
| Commitment | 181 | 183 |
| Carrying Value of Nonredeemable Funds by Contractual Maturity | ||
| Less than 5 years | 60 | |
| 5-10 years | 527 | |
| Over 10 years | 209 | |
| Total | 796 | |
| Hedge Funds | ||
| Measured Based on Net Asset Value | ||
| Carrying value | 99 | 90 |
| Commitment | $ 4 | $ 4 |