SEGMENT AND GEOGRAPHIC INFORMATION - Revenue by Product and Service Offering (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 26, 2020	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Segment Reporting Information [Line Items]
Revenue	$ 790,990	$ 743,300	$ 682,584	$ 707,059	$ 691,138	$ 667,951	$ 657,568	$ 604,569	$ 2,923,933	$ 2,621,226	$ 2,266,096
RMS
Segment Reporting Information [Line Items]
Revenue									571,152	537,089	519,682
DSA
Segment Reporting Information [Line Items]
Revenue									1,837,428	1,618,995	1,316,854
Manufacturing
Segment Reporting Information [Line Items]
Revenue									$ 515,353	$ 465,142	$ 429,560