Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 27, 2020	Dec. 29, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 765,104	$ 797,623
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	274,432	271,055
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	355,241	388,158
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	135,431	138,410
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	124,966	123,836
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	265,864	245,303
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	157,806	181,491
International
Disaggregation of Revenue [Line Items]
Revenue	$ 216,468	$ 246,993