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Concentration Risk - Narrative (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Customer Concentration Risk | The Bank of New York Mellon Corporation        
Concentration Risk [Line Items]        
Concentration risk 12.00% 16.00% 13.00% 16.00%