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Summary of Accounting Policies Summary of Accounting Policies (Management Compensation Plans Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Jan. 01, 2017
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Document Period End Date     Dec. 31, 2019    
Common stock, par value   $ 0.0001 $ 0.0001 $ 0.0001  
Vesting period     3 years    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage   200.00%      
Performance Shares [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Range of number of shares that will vest     zero to stretch    
Director [Member] | RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period     1 year    
Employee [Member] | RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period     3 years    
Retained Earnings [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Cumulative Effect on Retained Earnings, Net of Tax $ 1   $ 0 $ 0 $ (1)