Reportable Segments - Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jun. 30, 2025	Sep. 28, 2025	Jun. 29, 2025	Mar. 30, 2025	Dec. 29, 2024	Sep. 29, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 28, 2025	Sep. 29, 2024	Oct. 01, 2023
Financial information concerning reportable segments
Revenue		$ 1,330,100	$ 1,369,816	$ 1,322,113	$ 1,420,561	$ 1,374,473	$ 1,344,323	$ 1,251,616	$ 1,228,267	$ 5,442,590	$ 5,198,679	$ 4,522,550
Total consolidated subcontractor costs										825,230	876,817	771,461
Other segment items										(3,919,944)	(3,712,326)	(3,275,577)
Segment operating income		$ 181,304	$ 164,986	$ 39,603	22,526	$ 143,343	$ 128,630	$ 117,683	$ 111,081	408,419	500,737	358,113
Reconciliation of profit (segment operating income):
Legal contingency costs					$ (115,000)					(115,000)	0	0
Acquisition and integration expenses											(7,138)	(28,105)
Right-of-use operating lease asset impairment										0	0	(16,385)
Impairment of goodwill	$ 0									(92,416)	0	0
Contingent consideration – fair value adjustments										12,228	(2,541)	(12,255)
Interest income										9,837	7,288	5,898
Interest expense										(40,639)	(44,559)	(52,435)
Other Expenses										(93,809)	(99,120)	(75,881)
Other non-operating income										0	0	89,402
Income before income tax expense										377,617	463,466	400,978
Depreciation expense related to property and equipment										21,200	23,700	20,000
Acquisition and integration expenses										0	7,138	33,169
Amortization expense for intangible assets										37,100	50,000	41,200
Stock-based compensation expense										33,946	31,155	28,607
GSG
Financial information concerning reportable segments
Other segment items										(1,789,231)	(1,628,952)	(1,403,678)
Reconciliation of profit (segment operating income):
Impairment of goodwill										(92,416)
Depreciation expense related to property and equipment										3,000	3,300	3,300
Income (loss) from equity method investments										1,100	1,600	2,800
CIG
Financial information concerning reportable segments
Other segment items										(2,130,713)	(2,083,374)	(1,871,899)
Reconciliation of profit (segment operating income):
Impairment of goodwill										0
Depreciation expense related to property and equipment										17,800	20,100	16,300
Income (loss) from equity method investments										2,500	3,300	3,200
Acquisition and integration expenses												5,100
Operating Segments Excluding Intersegment Elimination
Financial information concerning reportable segments
Revenue										5,442,590	5,198,679	4,522,550
Total consolidated subcontractor costs										825,230	876,817	771,461
Operating Segments Excluding Intersegment Elimination | GSG
Financial information concerning reportable segments
Revenue										2,637,426	2,445,746	2,128,330
Total consolidated subcontractor costs										504,644	539,579	493,020
Operating Segments Excluding Intersegment Elimination | CIG
Financial information concerning reportable segments
Revenue										2,805,164	2,752,933	2,394,220
Total consolidated subcontractor costs										320,586	337,238	278,441
Intersegment Eliminations
Financial information concerning reportable segments
Revenue										(75,966)	(71,407)	(60,988)
Total consolidated subcontractor costs										75,966	71,407	60,988
Intersegment Eliminations | GSG
Financial information concerning reportable segments
Revenue										(36,483)	(37,609)	(30,559)
Total consolidated subcontractor costs										39,483	33,798	30,429
Intersegment Eliminations | CIG
Financial information concerning reportable segments
Revenue										(39,483)	(33,798)	(30,429)
Total consolidated subcontractor costs										36,483	37,609	30,559
Operating Segments
Financial information concerning reportable segments
Revenue										5,518,556	5,270,086	4,583,538
Total consolidated subcontractor costs										901,196	948,224	832,449
Segment operating income										697,416	609,536	475,512
Reconciliation of profit (segment operating income):
Right-of-use operating lease asset impairment												(15,100)
Operating Segments | GSG
Financial information concerning reportable segments
Revenue										2,673,909	2,483,355	2,158,889
Total consolidated subcontractor costs										544,127	573,377	523,449
Segment operating income										340,551	281,026	231,762
Reconciliation of profit (segment operating income):
Right-of-use operating lease asset impairment												(6,800)
Operating Segments | CIG
Financial information concerning reportable segments
Revenue										2,844,647	2,786,731	2,424,649
Total consolidated subcontractor costs										357,069	374,847	309,000
Segment operating income										356,865	328,510	243,750
Reconciliation of profit (segment operating income):
Right-of-use operating lease asset impairment												(8,300)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment
Reconciliation of profit (segment operating income):
Right-of-use operating lease asset impairment												(1,158)
Corporate
Reconciliation of profit (segment operating income):
Amortization expense for intangible assets										37,100	50,000	41,200
Stock-based compensation expense										$ 19,300	$ 18,500	$ 16,400