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Stock-Based Compensation Stock Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Unrecognized compensation cost $ 62   $ 62  
Remaining weighted average period     1 year 2 months  
Stock-based compensation expense $ 13 $ 15 $ 25 $ 26