Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Apr. 01, 2018	Mar. 31, 2019	Apr. 01, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 722,621	$ 700,262	$ 1,440,052	$ 1,460,010
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	129,868	104,905	253,147	256,659
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	216,498	236,951	441,256	473,199
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	165,970	180,398	338,758	387,184
International
Disaggregation of Revenue [Line Items]
Revenue	210,285	178,008	406,891	342,968
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	247,831	224,878	488,764	460,298
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	345,626	333,590	682,163	710,362
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	$ 129,164	$ 141,794	$ 269,125	$ 289,350