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Revenue - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Revenue from Contract with Customer [Abstract]    
Unbilled service fees $ 87,054 $ 64,362
Unbilled outside vendor costs, billable to clients 46,663 42,369
Advance billings 171,151 $ 138,505
Increase in advance billings 32,646  
Revenue recognized $ 60,941