Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies (Narrative) (Details)

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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
 3 Months Ended
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Accrued expenses and other current liabilities   $ 226,000,000 $ 279,000,000  
Accrued expenses and other current liabilities   242,000,000 220,000,000  
Cumulative-effect benefit to retained earnings   (82,000,000) 113,000,000  
Cumulative Effect, Period of Adoption, Adjustment        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative-effect benefit to retained earnings       $ 15,000,000
Deferred loyalty program revenue        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Loyalty program liability   81,000,000 90,000,000  
Accrued expenses and other current liabilities   43,000,000 56,000,000  
Accrued expenses and other current liabilities   $ 38,000,000 $ 34,000,000  
Cumulative adjustment, increase in loyalty revenues $ 16,000,000