Fair Value of Assets and Liabilities (Nonrecurring Fair Value Measurements) (Details) - Fair Value, Measurements, Nonrecurring - Level 3 - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Commercial mortgage loans | |||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Realized investment gains (losses) net | $ (2) | $ (5) | $ 0 |
| Carrying value after measurement as of period end | 64 | 47 | 0 |
| Mortgage servicing rights | |||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Realized investment gains (losses) net | 7 | (1) | (7) |
| Carrying value after measurement as of period end | 60 | 84 | 90 |
| Cost method investments | |||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | |||
| Realized investment gains (losses) net | (29) | (85) | (123) |
| Carrying value after measurement as of period end | $ 150 | $ 284 | $ 239 |