Restructuring and Other Charges - Schedule of Restructuring and Other Activities by Major Type of Costs (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 26, 2025	Sep. 27, 2024	Sep. 26, 2025	Sep. 27, 2024	Sep. 29, 2023
Interest Rate Swap
Restructuring Cost and Reserve [Line Items]
Derivative, gain on derivative	$ 35,200	$ 35,200		$ 35,200
CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring costs			$ 61,316	134,911	$ 126,219
Lease Abandonments and Impairments | CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring costs			0	49	44,788
Voluntary and Involuntary Terminations | CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring costs			29,753	47,881	37,235
Outside Services | CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring costs			24,651	100,593	35,099
Other | CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring costs			$ 6,912	$ (13,612)	$ 9,097