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Stock-Based Compensation Stock Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Unrecognized compensation cost $ 69  
Remaining weighted average period 1 year 5 months  
Stock-based compensation expense $ 12 $ 12