Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 3,107,348	$ 2,964,148	$ 2,753,360
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	587,364	469,231	353,062
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	941,102	974,384	901,136
U.S commercial
Disaggregation of Revenue [Line Items]
Revenue	719,314	788,398	764,643
International
Disaggregation of Revenue [Line Items]
Revenue	859,568	732,135	734,519
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	1,048,157	986,910	909,197
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	1,509,901	1,395,148	1,264,546
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	$ 549,290	$ 582,090	$ 579,617