Segment Information - Schedule of Total Revenues, Segment Operating Profit and Total Asset for Reporting Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2021	Jan. 01, 2021
Segment Reporting Information [Line Items]
Revenues	$ 3,380,625	$ 3,381,836
Total Segment Operating Profit	177,333	213,940
Restructuring, Transaction and Other Charges	(83,566)	(22,091)
Total Other (Expense) Income, net	(8,243)	140,171
Earnings from Continuing Operations Before Taxes	169,090	354,111
Amortization of intangible assets	46,907
Impairment of long-lived assets and equity method investment	72,266	27,902
Worley
Segment Reporting Information [Line Items]
Fair value adjustments		93,100
C3
Segment Reporting Information [Line Items]
Fair value adjustments		82,600
AWE Management Ltd
Segment Reporting Information [Line Items]
Impairment of long-lived assets and equity method investment		27,900
Gain on termination of lease	6,900
Fair value adjustments	1,700
Operating Segments
Segment Reporting Information [Line Items]
Total Segment Operating Profit	366,259	306,372
Corporate
Segment Reporting Information [Line Items]
Other Corporate Expenses	(105,360)	(70,341)
Corporate | Other Expense
Segment Reporting Information [Line Items]
Amortization of intangible assets	46,900	23,200
Critical Mission Solutions | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,162,505	1,295,287
Total Segment Operating Profit	111,496	110,072
People & Places Solutions | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,928,146	2,086,549
Total Segment Operating Profit	191,692	196,300
PA Consulting | Operating Segments
Segment Reporting Information [Line Items]
Revenues	289,974	0
Total Segment Operating Profit	$ 63,071	$ 0