Segment Reporting - Information by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 31, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 787,833	$ 666,240	$ 643,303	$ 617,253	$ 697,085	$ 697,717	$ 695,607	$ 673,484	$ 2,714,628	$ 2,763,893	$ 2,608,562
Operating income (loss)	$ 20,482	$ 28,532	$ 34,295	$ 7,457	$ (3,167)	$ (10,525)	$ 26,158	$ 14,353	90,766	26,819	50,121
Amortization of intangible assets									34,154	33,205	35,780
Corporate
Segment Reporting Information [Line Items]
Revenues									0	0	0
Operating income (loss)									(11,397)	(28,627)	(26,464)
Amortization of intangible assets									0	0	0
GMS | Operating Segments
Segment Reporting Information [Line Items]
Revenues									2,080,474	2,099,737	1,968,843
Operating income (loss)									80,090	92,386	89,141
Amortization of intangible assets									16,461	16,679	18,492
NSS | Operating Segments
Segment Reporting Information [Line Items]
Revenues									634,154	664,156	639,719
Operating income (loss)									22,073	(36,940)	(12,556)
Amortization of intangible assets									$ 17,693	$ 16,526	$ 17,288