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Stockholders' Equity - Components of Share-based Compensation Expenses (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock based compensation expense      
Pretax Charges $ 46 $ 26 $ 23
Net-of-Tax Amount 40 22 16
Restricted stock and performance awards      
Stock based compensation expense      
Pretax Charges 39 20 19
Net-of-Tax Amount 35 17 13
Employee stock purchase plan and stock options      
Stock based compensation expense      
Pretax Charges 7 6 4
Net-of-Tax Amount $ 5 $ 5 $ 3