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Stock Compensation - Performance Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2017
Feb. 28, 2017
2014 Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Performance share award settlement as a percentage of target   35.00%
Performance share award settlement   $ 5
2015 Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Maximum fair value of performance shares $ 20  
2016 Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Maximum fair value of performance shares $ 22  
2017 Award    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Percentage of performance shares that may be earned upon performance, minimum 0.00%  
Percentage of performance shares that may be earned upon performance, maximum 200.00%  
Performance shares target vesting period 3 years  
Performance shares target grant distribution (in shares) 186,000  
Grant-date fair value of shares that correspond with target grants $ 67  
Maximum fair value of performance shares $ 23