Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 29, 2020	Mar. 31, 2019	Mar. 29, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue	$ 734,133	$ 722,621	$ 1,531,756	$ 1,440,052
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	255,613	247,831	526,668	488,764
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	350,618	345,626	738,776	682,163
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	127,902	129,164	266,312	269,125
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	102,260	129,868	224,616	253,147
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	243,261	216,498	488,564	441,256
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	162,198	165,970	345,169	338,758
International
Disaggregation of Revenue [Line Items]
Revenue	$ 226,414	$ 210,285	$ 473,407	$ 406,891