Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 30, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 717,431	$ 759,749
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	123,280	151,754
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	224,757	236,249
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	172,788	206,786
International
Disaggregation of Revenue [Line Items]
Revenue	196,606	$ 164,960
GSG
Disaggregation of Revenue [Line Items]
Revenue	717,431
GSG | Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	240,933
GSG | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	336,537
GSG | Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	139,961
CIG
Disaggregation of Revenue [Line Items]
Revenue	759,749
CIG | Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	235,420
CIG | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	376,773
CIG | Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	$ 147,556