Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 29, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 27, 2020	Sep. 29, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 753,364	$ 709,771	$ 734,133	$ 797,623	$ 841,502	$ 825,793	$ 722,621	$ 717,431	$ 2,994,891	$ 3,107,348	$ 2,964,148
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue									1,078,432	1,048,157	986,910
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue									1,391,592	1,509,901	1,395,148
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue									524,867	549,290	582,090
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue									439,019	587,364	469,231
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue									993,835	941,102	974,384
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue									674,605	719,314	788,398
International
Disaggregation of Revenue [Line Items]
Revenue									$ 887,432	$ 859,568	$ 732,135