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Note 12. Stock-Based Compensation Stock-Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Unrecognized compensation cost $ 36    
Remaining weighted average period 1 year 11 months    
Stock-based compensation expense $ 52 $ 57 $ 57