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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]          
Valuation allowance, decrease     $ 2    
Valuation allowance   $ 103 101 $ 103  
Income tax provision from change in valuation allowance     1    
Tax Act, provisional income tax benefit   275   275  
Tax Act, measurement period adjustment, tax expense $ 19   19    
Unrecognized tax benefits that would impact effective tax rate     35    
Expected significant change in unrecognized tax benefits     3    
Interest expense and penalties     1 1 $ 1
Accrued interest expense and penalties   $ 3 4 $ 3  
Federal          
Income Tax Contingency [Line Items]          
Net operating loss carry-forwards     27    
State          
Income Tax Contingency [Line Items]          
Net operating loss carry-forwards     479    
Foreign tax authority          
Income Tax Contingency [Line Items]          
Net operating loss carry-forwards     42    
Net expiring and not expiring operating loss carry-forwards     $ 465