Reportable Segments - Revenue By Client Sector (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Jan. 01, 2017
Revenue by client sector
Threshold percentage for disclosure of revenue from a single client	10.00%	10.00%
Revenue	$ 759,749	$ 668,851
U.S. federal government
Revenue by client sector
Revenue	236,249	222,634
International
Revenue by client sector
Revenue	164,960	172,457
U.S. state and local government
Revenue by client sector
Revenue	151,754	83,494
U.S. commercial
Revenue by client sector
Revenue	$ 206,786	$ 190,266