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Overview and Summary of Significant Accounting Policies - Customer Accounts (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Mar. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Minimum aggregate customer balances required to be covered by eligible liquid assets held, percentage 100.00%  
Europe    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Funds receivable and customer accounts designated for credit funding, percentage 30.00%  
U.S. | Minimum    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Funds receivable and funds payable, transaction clearing period 1 day  
U.S. | Maximum    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Funds receivable and funds payable, transaction clearing period 3 days  
Other countries    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Funds receivable and funds payable, transaction clearing period 5 days  
Cash and cash equivalents | Europe    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Funds receivable and customer accounts designated for credit funding $ 700 $ 800