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
Jun. 30, 2017
Dec. 31, 2016
Unrealized Losses    
Less Than 12 Months $ (20) $ (59)
12 Months or More (3) (1)
Fair Value    
Less Than 12 Months 2,156 3,218
12 Months or More 104 89
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 0 0
Fair Value    
Less Than 12 Months 263 215
12 Months or More 3 2
Corporate securities    
Unrealized Losses    
Less Than 12 Months (6) (12)
12 Months or More 0 (1)
Fair Value    
Less Than 12 Months 662 1,020
12 Months or More 22 39
Municipal securities    
Unrealized Losses    
Less Than 12 Months (8) (35)
12 Months or More 0 0
Fair Value    
Less Than 12 Months 743 1,423
12 Months or More 36 30
Asset-backed securities    
Unrealized Losses    
Less Than 12 Months 0 (1)
12 Months or More 0 0
Fair Value    
Less Than 12 Months 146 101
12 Months or More 13 18
Residential mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (3) (5)
12 Months or More (1) 0
Fair Value    
Less Than 12 Months 154 188
12 Months or More 16 0
Commercial mortgage-backed securities    
Unrealized Losses    
Less Than 12 Months (2) (5)
12 Months or More (2) 0
Fair Value    
Less Than 12 Months 188 271
12 Months or More $ 14 $ 0