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Revenue - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]    
Unbilled service fees $ 92,900 $ 54,177
Unbilled outside vendor costs, billable to clients 59,317 31,146
Advance billings 182,305 $ 148,133
Increase in advance billings 34,172  
Revenue recognized $ 85,665