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Overview and Summary of Significant Accounting Policies - Allowance for Transaction Losses and Negative Customer Balances (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Valuation and Qualifying Accounts Disclosure [Line Items]    
Allowance for negative customer balances $ 174 $ 144
Accrued expenses and other current liabilities    
Valuation and Qualifying Accounts Disclosure [Line Items]    
Allowance for transaction losses $ 92 $ 78