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Stock-Based Compensation Stock Options (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
4 Months Ended 8 Months Ended 12 Months Ended
Aug. 31, 2017
Dec. 31, 2017
Aug. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock Option Award Vesting Period 3 years          
Stock Option Award Requisite Service Period 6 months          
Grants between 2010 and 2015 [Member] | Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock Option, Expiration Period 7 years          
Grants in 2016 and 2017 [Member] | Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock Option, Expiration Period 10 years          
Successor [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Tax Benefit from Compensation Expense   $ 11        
Unrecognized Pretax Compensation Expense Related to Stock Options   $ 24   $ 24    
Successor [Member] | Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-Average Grant Date Fair Value, Stock Options   $ 28.56        
Total Intrinsic Value of Stock Options Exercised   $ 19        
Tax Benefit from Compensation Expense   $ 6        
Remaining Weighted-Average Recognition Period       1 year 314 days    
Predecessor [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Tax Benefit from Compensation Expense     $ 29   $ 39 $ 42
Predecessor [Member] | Employee Stock Option [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Weighted-Average Grant Date Fair Value, Stock Options     $ 16.65   $ 13.40 $ 11.57
Total Intrinsic Value of Stock Options Exercised     $ 108   $ 86 $ 160
Tax Benefit from Compensation Expense     $ 36