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ACCOUNTS RECEIVABLE AND SIGNIFICANT CLIENTS(NARRATIVE) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Entity Wide Revenue Major Customer Line Items                      
Revenue. $ 570,974 $ 492,980 $ 453,081 $ 432,213 $ 461,326 $ 395,507 $ 392,515 $ 394,356 $ 1,949,248 $ 1,643,704 $ 1,509,171
Sears                      
Entity Wide Revenue Major Customer Line Items                      
Accounts receivable amount from major customer $ 2,700               $ 2,700   $ 2,700
Customer Concentration Risk                      
Entity Wide Revenue Major Customer Line Items                      
Concentration risk percentage                 10.00%    
Customer Concentration Risk | Minimum                      
Entity Wide Revenue Major Customer Line Items                      
Revenue.                 $ 100,000    
Revenue | Customer Concentration Risk                      
Entity Wide Revenue Major Customer Line Items                      
Concentration Risk, Customer                 one no one
Revenue | Customer Concentration Risk | Financial Services Client                      
Entity Wide Revenue Major Customer Line Items                      
Concentration risk percentage                 13.00% 3.00% 4.00%
Revenue | Customer Concentration Risk | Healthcare client                      
Entity Wide Revenue Major Customer Line Items                      
Concentration risk percentage                 6.00% 8.00% 10.00%