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Revenue - Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Revenue from Contract with Customer [Abstract]    
Unbilled service fees $ 66,119 $ 64,362
Unbilled outside vendor costs, billable to clients 30,133 42,369
Advance billings 171,742 $ 138,505
Deferred Revenue, Additions 33,237  
Revenue recognized $ 121,659