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Stock-Based Compensation Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for future grants 15,725,806    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Unrecognized compensation cost $ 15    
Weighted average recognition period 1 year 9 months 18 days    
Intrinsic value of units outstanding $ 28    
Intrinsic value of units vested 26 $ 14 $ 18
Fair value of units vested $ 14 $ 21 $ 14
Shares granted (in shares) 1,995,473 826,546 809,436
Issued weighted average fair value (in dollars per shares) $ 13.24 $ 17.20 $ 21.15
Restricted Stock Units | Directors' Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 1 year    
Shares granted (in shares) 63,090 54,855  
Performance Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Shares granted (in shares) 1,073,934 889,599  
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 $ 4    
Weighted average recognition period 1 year 8 months 12 days    
Expiration period 10 years    
Options exercised 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 150,629 147,680  
Shares reserved for future purchase under the ESPP 2,770,428