Investments (Securities Pledged) (Details) - USD ($) $ in Millions |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | $ 16,836 | $ 15,626 |
| Fixed maturities | ||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | 13,303 | 11,393 |
| Trading account assets supporting insurance liabilities | ||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | 369 | 477 |
| Other trading account assets | ||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | 1 | 2 |
| Separate account assets | ||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | 2,992 | 3,386 |
| Equity securities | ||
| Financial Instruments Owned and Pledged as Collateral [Line Items] | ||
| Total securities pledged | $ 171 | $ 368 |