Goodwill and Intangible Assets - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 01, 2024 USD ($)	Feb. 28, 2025 USD ($)	Jun. 29, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 29, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jan. 20, 2025 contract	Sep. 29, 2024 USD ($)
Goodwill [Line Items]
Impairment of goodwill	$ 0		$ 0	$ 0	$ 92,416	$ 0
Percentage of excess of fair value over carrying value (less than)	72.00%
Percentage of cancellation programs							83.00%
Number of contracts cancelled | contract							5,200
Goodwill			2,084,872		2,084,872			$ 2,046,569
Amortization expense			8,300	$ 13,800	27,600	$ 38,400
GSG
Goodwill [Line Items]
Gross amounts of goodwill			770,200		770,200			768,500
Accumulated impairment			110,100		110,100			17,700
Impairment of goodwill					92,416
Goodwill			660,131		660,131			750,817
CIG
Goodwill [Line Items]
Gross amounts of goodwill			1,546,300		1,546,300			1,417,300
Accumulated impairment			121,500		121,500			121,500
Impairment of goodwill					0
Goodwill			1,424,741		1,424,741			$ 1,295,752
Global Development Services
Goodwill [Line Items]
Impairment of goodwill		$ 92,400	92,400
Goodwill		$ 130,500	$ 38,100		$ 38,100