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Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Debt Securities    
Amortized Cost $ 452 $ 341
Gross Unrealized Gains 8 1
Gross Unrealized Losses 0 0
Fair Value 460 342
Asset-backed securities    
Debt Securities    
Amortized Cost 22 30
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 22 30
Corporate bonds    
Debt Securities    
Amortized Cost 118 123
Gross Unrealized Gains 3 1
Gross Unrealized Losses 0 0
Fair Value 121 124
U.S. government agencies and government- sponsored agencies    
Debt Securities    
Amortized Cost 20 14
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 20 14
U.S. treasuries    
Debt Securities    
Amortized Cost 285 163
Gross Unrealized Gains 5 0
Gross Unrealized Losses 0 0
Fair Value 290 163
Certificate of deposits    
Debt Securities    
Amortized Cost 0 1
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 0 1
Other debt securities    
Debt Securities    
Amortized Cost 7 10
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value $ 7 $ 10