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Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]    
Customer Reward Program $ 5.4 $ 4.7
Loan origination fees and direct loan origination costs amortization period 12 months  
Minimum    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Partnership Agreement Initial Term 2 years  
Maximum    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Partnership Agreement Initial Term 10 years  
Credit Card    
Financing Receivable, Allowance for Credit Loss [Line Items]    
Loan origination fees and direct loan origination costs amortization period 12 months  
Amortization term of annual membership fees 12 months