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Stock-Based Compensation Stock Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Unrecognized compensation cost $ 59   $ 59  
Remaining weighted average period     1 year 4 months  
Stock-based compensation expense $ 14 $ 13 $ 26 $ 25