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Revenue - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Revenue from Contract with Customer [Abstract]    
Unbilled service fees $ 92,317 $ 64,362
Unbilled outside vendor costs, billable to clients 40,605 42,369
Advance billings 168,142 $ 138,505
Increase in advance billings 29,637  
Revenue recognized $ 101,431