Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Transfers Between Level 1 and Level 2) (Details) - Separate account assets - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||
| Fair Value Assets Level1 To Level2 Transfers | $ 111 | $ 86 |
| Fair Value Assets Level2 to Level1 Transfers | $ 207 | $ 40 |