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Stock-Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Payment Arrangement [Abstract]      
Unrecognized compensation cost $ 24    
Remaining weighted average period 1 year 6 months    
Stock-based compensation expense $ 39 $ 30 $ 40