REVENUE FROM CONTRACTS WITH CUSTOMERS - Schedule of Client Receivables, Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	Mar. 27, 2021	Dec. 26, 2020
Revenue from Contract with Customer [Abstract]
Client receivables	$ 463,800	$ 489,042
Contract assets (unbilled revenue)	154,044	135,400
Contract liabilities (current and long-term deferred revenue)	233,075	227,417
Contract liabilities (customer contract deposits)	$ 36,652	$ 42,244

Amount, before allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130533-203044

Name:	us-gaap_ContractWithCustomerAssetGrossCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant