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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Debt and Equity Securities |  |
Amortized Cost | $ 195 | $ 195
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 195 | 195
Asset-backed securities |  |
Debt Securities |  |
Amortized Cost | 35 | 33
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 35 | 33
Corporate bonds |  |
Debt Securities |  |
Amortized Cost | 100 | 99
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 100 | 99
U.S. government agencies and government- sponsored agencies |  |
Debt Securities |  |
Amortized Cost | 6 | 7
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 6 | 7
U.S. treasuries |  |
Debt Securities |  |
Amortized Cost | 47 | 46
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 47 | 46
Exchange traded fund |  |
Equity Securities |  |
Amortized Cost | 1 | 1
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 1 | 1
Other debt securities |  |
Debt Securities |  |
Amortized Cost | 6 | 9
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 6 | $ 9

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