Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 709,771	$ 825,793	$ 2,241,527	$ 2,265,846
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	275,810	271,287	802,477	760,051
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	309,123	419,564	1,047,900	1,101,728
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	124,838	134,942	391,150	404,067
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	96,669	172,148	321,286	425,296
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	243,414	245,723	731,977	686,979
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	168,985	194,980	514,154	533,738
International
Disaggregation of Revenue [Line Items]
Revenue	$ 200,703	$ 212,942	$ 674,110	$ 619,833