Consolidated Variable Interest Entities - Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Variable Interest Entity [Line Items]
Revenues	$ 666,240	$ 697,717	$ 1,926,795	$ 2,066,808
Cost of revenues	512,877	565,703	1,474,763	1,623,634
Selling, general and administrative expenses	119,168	133,215	361,945	394,689
Total operating expenses	640,092	707,094	1,860,849	2,043,352
Program profit (loss)	26,148	(9,377)	65,946	23,456
Other (income), net	2,384	(1,148)	4,338	6,530
Net income (loss)	10,318	(32,170)	21,395	(35,216)
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Revenues	87,324	95,121	268,969	248,170
Cost of revenues	70,707	106,560	215,866	228,513
Selling, general and administrative expenses	16,595	24,100	50,477	63,828
Total operating expenses	87,302	130,660	266,343	292,341
Program profit (loss)	22	(35,539)	2,626	(44,171)
Other (income), net	(32)	(237)	(241)	(1,090)
Net income (loss)	$ (10)	$ (35,776)	$ 2,385	$ (45,261)