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Investments - Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Investment [Line Items] |  |  |  |  |
Gross gains on fixed income securities | $ 21,000,000 |  | $ 95,000,000 |  |
Gross loss on fixed income securities | 48,000,000 |  | 242,000,000 |  |
Gross gains on fixed income and equity securities |  | $ 145,000,000 |  | $ 521,000,000 |
Gross loss on fixed income and equity securities |  | 36,000,000 |  | 161,000,000 |
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date | 195,000,000 |  | 195,000,000 |  | $ 208,000,000
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 862,000,000 |  | 862,000,000 |  |
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 16,000,000 |  | 16,000,000 |  |
Limited partnership interests | 7,602,000,000 |  | 7,602,000,000 |  | 6,740,000,000
Fixed income and equity securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Unrealized losses | 878,000,000 |  | 878,000,000 |  | 295,000,000
Investment grade fixed income securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 770,000,000 |  | 770,000,000 |  |
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 12,000,000 |  | 12,000,000 |  |
Below investment grade fixed income securities |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 92,000,000 |  | 92,000,000 |  |
Unrealized losses having loss of less than twenty percent, less than 12 months | 58,000,000 |  | 58,000,000 |  |
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 4,000,000 |  | 4,000,000 |  |
Unrealized losses related to securities with unrealized loss position greater than 20% of cost or amortized cost, unrealized loss position of 12 or more consecutive months | 1,000,000 |  | 1,000,000 |  |
EMA limited partnerships |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Limited partnership interests | 5,890,000,000 |  | 5,890,000,000 |  | 5,410,000,000
Carrying value | Cost-method Investments |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Cost method limited partnerships | 1,710,000,000 |  | 1,710,000,000 |  | $ 1,330,000,000
Mortgage loans, non-impaired |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Allowance for credit losses, period increase (decrease) | $ 0 | $ 0 | 0 | 0 |
Average impaired mortgage loans |  |  | $ 4,000,000 | $ 8,000,000 |

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