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Short-Term and Long-Term Investments - Schedule of Available for Sale Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Common stock, shares, issued (in shares) 600,000,000 596,000,000
Cost   $ 347
Gross Unrealized Gains   108
Gross Unrealized Losses   0
Estimated Fair Value   455
Mutual funds    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 16 23
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 16 23
Cetip equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost   324
Gross Unrealized Gains   108
Gross Unrealized Losses   0
Estimated Fair Value   $ 432