Restructuring and Other Charges - Schedule of Restructuring and Other Charges Impacts on Reportable Segment Income by Line of Business (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
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 61,316
AWE ML
Restructuring Cost and Reserve [Line Items]
Gain on termination of lease					$ 3,400
Interest Rate Swap
Restructuring Cost and Reserve [Line Items]
Derivative, gain on derivative	$ 35,200	$ 35,200		$ 35,200
Operating Segments | Employee Separation Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges				6,400
CH2M HILL Companies, Ltd.
Restructuring Cost and Reserve [Line Items]
Restructuring charges			61,316	134,911	126,219
Impairment of real estate including other moving costs			49,100
CH2M HILL Companies, Ltd. | SG&A Expense
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 61,316	$ 169,844	$ 129,596
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]			Operating Profit	Operating Profit	Selling, General and Administrative Expense
CH2M HILL Companies, Ltd. | Other (Income), net
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 0	$ (34,933)	$ (3,377)
Restructuring Charges, Statement of Income or Comprehensive Income [Extensible Enumeration]			Nonoperating Income (Expense)	Nonoperating Income (Expense)	Nonoperating Income (Expense)
CH2M HILL Companies, Ltd. | Professional Services and Employee Separation Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 58,800	$ 163,400	$ 61,100
CH2M HILL Companies, Ltd. | Employee Separation Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges					14,300
CH2M HILL Companies, Ltd. | PA Consulting | Employee Separation Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges			2,200	6,400	14,300
CH2M HILL Companies, Ltd. | Operating Segments | Infrastructure & Advanced Facilities
Restructuring Cost and Reserve [Line Items]
Restructuring charges			59,092	128,529	111,513
CH2M HILL Companies, Ltd. | Operating Segments | PA Consulting
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 2,224	$ 6,382	$ 14,706