v3.8.0.1
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, 2017
Dec. 31, 2016
Unrealized Losses    
Less than 12 months $ (17) $ (59)
12 Months or longer (14) (1)
Fair Value    
Less than 12 months 2,658 3,218
12 Months or longer 689 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 longer (1) 0
Fair Value    
Less than 12 months 222 215
12 Months or longer 79 2
Corporate securities    
Unrealized Losses    
Less than 12 months (6) (12)
12 Months or longer (4) (1)
Fair Value    
Less than 12 months 1,044 1,020
12 Months or longer 185 39
Municipal securities    
Unrealized Losses    
Less than 12 months (7) (35)
12 Months or longer (3) 0
Fair Value    
Less than 12 months 943 1,423
12 Months or longer 175 30
Asset-backed securities    
Unrealized Losses    
Less than 12 months (1) (1)
12 Months or longer 0 0
Fair Value    
Less than 12 months 228 101
12 Months or longer 28 18
Residential mortgage-backed securities    
Unrealized Losses    
Less than 12 months (1) (5)
12 Months or longer (5) 0
Fair Value    
Less than 12 months 109 188
12 Months or longer 171 0
Commercial mortgage- backed securities    
Unrealized Losses    
Less than 12 months (1) (5)
12 Months or longer (1) 0
Fair Value    
Less than 12 months 112 271
12 Months or longer 51 0
Investments    
Schedule of Available-for-sale Securities [Line Items]    
Investments And Restricted Deposits Contractual Maturities Within One Year Fair Value $ 474 $ 500