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Stock-Based Compensation Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for future grants 20,581,676    
Intrinsic value of options outstanding $ 5    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Unrecognized compensation cost $ 10    
Weighted average recognition period 1 year 7 months 6 days    
Intrinsic value of units outstanding $ 40    
Fair value of units vested $ 6 $ 8 $ 11
Shares granted (in shares) 4,123,544 3,113,886 1,754,098
Issued weighted average fair value (in dollars per shares) $ 3.92 $ 6.56 $ 12.36
Restricted Stock Units | Directors' Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 1 year    
Shares granted (in shares) 282,131 155,709  
Performance Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Shares granted (in shares) 0 1,368,182 733,148
Issued weighted average fair value (in dollars per shares) $ 0 $ 6.60 $ 12.64
Performance Stock Units | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of target number of awards granted 0.00%    
Performance Stock Units | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of target number of awards granted 200.00%    
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost $ 2    
Weighted average recognition period 1 year 6 months    
Expiration period 10 years    
Options exercised (in shares) 33,501 0 0
Stock Options | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Employee Stock Purchase Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee stock purchase plan discounted offering price of commons stock 95.00%    
Shares purchased under the Employee Stock Purchase Plan (in shares) 291,540 258,838  
Shares reserved for future purchase under the ESPP (in shares) 2,001,626