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Short-term and Long-term Investments, Restricted Deposits - Fair Value of Available-for-Sale Investments in a Continuous Unrealized Loss Position (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Unrealized Losses    
Less Than 12 Months $ (36) $ (17)
12 Months or More (55) (14)
Fair Value    
Less Than 12 Months 2,555 2,658
12 Months or More 2,420 689
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Unrealized Losses    
Less Than 12 Months 0 (1)
12 Months or More (2) (1)
Fair Value    
Less Than 12 Months 59 222
12 Months or More 202 79
Corporate securities    
Unrealized Losses    
Less Than 12 Months (27) (6)
12 Months or More (25) (4)
Fair Value    
Less Than 12 Months 1,389 1,044
12 Months or More 871 185
Municipal securities    
Unrealized Losses    
Less Than 12 Months (4) (7)
12 Months or More (14) (3)
Fair Value    
Less Than 12 Months 591 943
12 Months or More 806 175
Asset-backed securities    
Unrealized Losses    
Less Than 12 Months (2) (1)
12 Months or More (2) 0
Fair Value    
Less Than 12 Months 318 228
12 Months or More 168 28
Residential mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (1) (1)
12 Months or More (8) (5)
Fair Value    
Less Than 12 Months 61 109
12 Months or More 233 171
Commercial mortgage- backed securities    
Unrealized Losses    
Less Than 12 Months (2) (1)
12 Months or More (4) (1)
Fair Value    
Less Than 12 Months 137 112
12 Months or More $ 140 $ 51