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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Debt and Equity Securities |  |
Amortized Cost | $ 174 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 174 |
Available-for-Sale Securities |  |
Amortized Cost |  | $ 38
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Fair Value |  | 38
Asset-backed securities |  |
Debt Securities |  |
Amortized Cost | 35 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 35 |
Corporate bonds |  |
Debt Securities |  |
Amortized Cost | 86 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 86 |
U.S. government agencies and government- sponsored agencies |  |
Debt Securities |  |
Amortized Cost | 8 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 8 |
U.S. treasuries |  |
Debt Securities |  |
Amortized Cost | 34 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 34 |
Available-for-Sale Securities |  |
Amortized Cost |  | 37
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Fair Value |  | 37
Exchange traded fund |  |
Equity Securities |  |
Amortized Cost | 1 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 1 |
Available-for-Sale Securities |  |
Amortized Cost |  | 1
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Fair Value |  | $ 1
Other debt securities |  |
Debt Securities |  |
Amortized Cost | 10 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | $ 10 |

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