Reportable Segments - Revenue by Sector (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Oct. 02, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 27, 2015	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 28, 2014	Oct. 02, 2016	Sep. 27, 2015	Sep. 28, 2014
Revenue by client sector
Threshold percentage for disclosure of revenue from a single client									10.00%	10.00%	10.00%
Revenue	$ 728,508	$ 666,869	$ 627,384	$ 560,708	$ 578,394	$ 575,108	$ 564,763	$ 581,056	$ 2,583,469	$ 2,299,321	$ 2,483,814
International
Revenue by client sector
Revenue									724,918	564,882	643,649
U.S. commercial
Revenue by client sector
Revenue									763,443	736,815	713,266
U.S. federal government
Revenue by client sector
Revenue									784,368	709,600	772,290
U.S. state and local government
Revenue by client sector
Revenue									$ 310,740	$ 288,024	$ 354,609