Segment Reporting (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Segment Reporting Information [Line Items]
Revenues	$ 666,240	$ 697,717	$ 1,926,795	$ 2,066,808
Operating income	28,532	(10,525)	70,284	29,986
Amortization of intangible assets	8,047	8,176	24,141	25,029
Corporate
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Operating income	(8,548)	(10,123)	(23,027)	(24,193)
Amortization of intangible assets	0	0	0	0
GMS \| Operating Segments
Segment Reporting Information [Line Items]
Revenues	521,346	535,635	1,486,643	1,566,138
Operating income	31,401	26,000	75,541	77,449
Amortization of intangible assets	4,115	4,140	12,346	12,539
NSS \| Operating Segments
Segment Reporting Information [Line Items]
Revenues	144,894	162,082	440,152	500,670
Operating income	5,679	(26,402)	17,770	(23,270)
Amortization of intangible assets	$ 3,932	$ 4,036	$ 11,795	$ 12,490

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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