Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 28, 2021	Mar. 29, 2020	Mar. 28, 2021	Mar. 29, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 754,764	$ 734,133	$ 1,519,868	$ 1,531,756
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	272,118	255,613	546,551	526,668
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	347,197	350,618	702,438	738,776
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	135,449	127,902	270,879	266,312
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	129,781	103,671	254,747	227,507
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	267,696	243,261	533,561	488,564
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	143,702	160,787	301,508	342,278
International
Disaggregation of Revenue [Line Items]
Revenue	$ 213,585	$ 226,414	$ 430,052	$ 473,407