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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Debt Securities |  |
Amortized Cost | $ 440 | $ 444
Gross Unrealized Gains | 5 | 7
Gross Unrealized Losses | 0 | 0
Fair Value | 445 | 451
Asset-backed securities |  |
Debt Securities |  |
Amortized Cost | 25 | 24
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 25 | 24
Corporate bonds |  |
Debt Securities |  |
Amortized Cost | 121 | 126
Gross Unrealized Gains | 2 | 2
Gross Unrealized Losses | 0 | 0
Fair Value | 123 | 128
U.S. government agencies and government- sponsored agencies |  |
Debt Securities |  |
Amortized Cost | 23 | 27
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | 0 | 0
Fair Value | 23 | 28
U.S. treasuries |  |
Debt Securities |  |
Amortized Cost | 256 | 261
Gross Unrealized Gains | 3 | 4
Gross Unrealized Losses | 0 | 0
Fair Value | 259 | 265
Certificate of deposits |  |
Debt Securities |  |
Amortized Cost | 9 | 0
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 9 | 0
Other debt securities |  |
Debt Securities |  |
Amortized Cost | 6 | 6
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 6 | $ 6

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