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Summary of Significant Accounting Policies - Customer Rewards Reserve and Revenue Recognition (Details) - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]    
Customer Reward Program $ 4.7 $ 4.3
Minimum    
Loans and Leases Receivable Disclosure [Line Items]    
Partnership Agreement Initial Term 2 years  
Maximum    
Loans and Leases Receivable Disclosure [Line Items]    
Partnership Agreement Initial Term 10 years  
Credit Card    
Loans and Leases Receivable Disclosure [Line Items]    
Loan Commitment and Origination Fees and Discounts or Premiums, Amortization Period 12 months  
Amortization term of annual membership fees 12 months