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Investments - Available-for-Sale Investments (Details) $ in Millions | Jun. 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 | $ 1,354 | $ 1,940
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 10 | $ 5
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 725 | 719
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 269 | $ 227
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 4 | $ 3
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 180 | 164
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 959 | $ 1,297
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 7 | $ 3
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 535 | 561
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 116 | $ 94
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 | 72 | 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 | $ 224 | $ 470
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses | $ 1 | $ 1
In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) | Security | 52 | 89
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 0 | $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses | $ 0 | $ 0
In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) | Security | 0 | 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 | $ 113 | $ 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 | 56 | 47
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value |  | $ 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 | 79 | 0
In a Continuous Loss Position for 12 Months or More, Estimated Fair value | $ 40 | $ 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 | 52 | 48
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value | $ 90 |
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 | 59 |
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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