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
Mar. 31, 2018
Dec. 31, 2017
Unrealized Losses    
Less Than 12 Months $ (59) $ (17)
12 Months or More (24) (14)
Fair Value    
Less Than 12 Months 3,877 2,658
12 Months or More 679 689
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Unrealized Losses    
Less Than 12 Months (2) (1)
12 Months or More (1) (1)
Fair Value    
Less Than 12 Months 171 222
12 Months or More 85 79
Corporate securities    
Unrealized Losses    
Less Than 12 Months (26) (6)
12 Months or More (7) (4)
Fair Value    
Less Than 12 Months 1,598 1,044
12 Months or More 185 185
Municipal securities    
Unrealized Losses    
Less Than 12 Months (22) (7)
12 Months or More (6) (3)
Fair Value    
Less Than 12 Months 1,467 943
12 Months or More 171 175
Asset-backed securities    
Unrealized Losses    
Less Than 12 Months (2) (1)
12 Months or More (1) 0
Fair Value    
Less Than 12 Months 310 228
12 Months or More 27 28
Residential mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (3) (1)
12 Months or More (7) (5)
Fair Value    
Less Than 12 Months 159 109
12 Months or More 161 171
Commercial mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (4) (1)
12 Months or More (2) (1)
Fair Value    
Less Than 12 Months 172 112
12 Months or More $ 50 $ 51