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Investments - Available-for-Sale Investments (Details) $ in Millions | Sep. 30, 2018 USD ($) Security | Dec. 31, 2017 USD ($) Security
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 862 | $ 1,940
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 6 | $ 5
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 564 | 719
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 462 | $ 227
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 7 | $ 3
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 283 | 164
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 727 | $ 1,297
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 4 | $ 3
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 460 | 561
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 186 | $ 94
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 3 | $ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 127 | 69
U.S. treasury notes |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 0 | $ 470
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 0 | $ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 0 | 89
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 94 | $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 1 | $ 0
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 31 | 0
GSEs |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 0 | $ 173
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 0 | $ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 0 | 69
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 127 | $ 95
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 2 | $ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 68 | 47
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 63 | $ 0
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 1 | $ 0
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 63 | 0
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 55 | $ 38
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 1 | $ 1
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 57 | 48
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 72 |
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 1 |
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 41 |
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 0 |
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 0 |
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 0 |

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