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Investments - Investments With a Continuous Realized Loss Position (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Fair Value    
Less than 12 months $ 135 $ 5
12 months or more 10 24
Total 145 29
Unrealized Losses    
Less than 12 months 0 0
12 months or more 0 0
Total 0 0
Asset-backed securities    
Fair Value    
Less than 12 months 26  
12 months or more 0  
Total 26  
Unrealized Losses    
Less than 12 months 0  
12 months or more 0  
Total 0  
Corporate bonds    
Fair Value    
Less than 12 months 82  
12 months or more 0  
Total 82  
Unrealized Losses    
Less than 12 months 0  
12 months or more 0  
Total 0  
U.S. government agencies and government- sponsored agencies    
Fair Value    
Less than 12 months 5  
12 months or more 0  
Total 5  
Unrealized Losses    
Less than 12 months 0  
12 months or more 0  
Total 0  
U.S. treasuries    
Fair Value    
Less than 12 months 20 5
12 months or more 10 24
Total 30 29
Unrealized Losses    
Less than 12 months 0 0
12 months or more 0 0
Total 0 $ 0
Other debt securities    
Fair Value    
Less than 12 months 2  
12 months or more 0  
Total 2  
Unrealized Losses    
Less than 12 months 0  
12 months or more 0  
Total $ 0