Quarterly report pursuant to Section 13 or 15(d)

Short-term and Long-term Investments, Restricted Deposits - Fair Value of Available-For-Sale Investments with Gross Unrealized Losses by Investment Type and Length of Time (Details)

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Short-term and Long-term Investments, Restricted Deposits - Fair Value of Available-For-Sale Investments with Gross Unrealized Losses by Investment Type and Length of Time (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Unrealized Losses    
Less Than 12 Months $ (68) $ (17)
12 Months or More (40) (14)
Fair Value    
Fair Value, Less Than 12 Months 4,293 2,658
Fair Value, 12 Months or More 1,118 689
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Unrealized Losses    
Less Than 12 Months (1) (1)
12 Months or More (2) (1)
Fair Value    
Fair Value, Less Than 12 Months 93 222
Fair Value, 12 Months or More 161 79
Corporate securities    
Unrealized Losses    
Less Than 12 Months (31) (6)
12 Months or More (13) (4)
Fair Value    
Fair Value, Less Than 12 Months 1,913 1,044
Fair Value, 12 Months or More 337 185
Municipal securities    
Unrealized Losses    
Less Than 12 Months (25) (7)
12 Months or More (10) (3)
Fair Value    
Fair Value, Less Than 12 Months 1,518 943
Fair Value, 12 Months or More 286 175
Asset-backed securities    
Unrealized Losses    
Less Than 12 Months (3) (1)
12 Months or More (1) 0
Fair Value    
Fair Value, Less Than 12 Months 409 228
Fair Value, 12 Months or More 66 28
Residential mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (4) (1)
12 Months or More (10) (5)
Fair Value    
Fair Value, Less Than 12 Months 186 109
Fair Value, 12 Months or More 178 171
Commercial mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (4) (1)
12 Months or More (4) (1)
Fair Value    
Fair Value, Less Than 12 Months 174 112
Fair Value, 12 Months or More $ 90 $ 51