Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Sep. 29, 2018	Sep. 30, 2017	Oct. 01, 2016	Oct. 03, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 272,940	$ 208,398	$ 186,720	$ 468,950	$ 214,095	$ 223,078	$ 182,546	$ 372,807	$ 1,137,008	$ 992,526	$ 901,284	$ 843,524
Gross profit	116,277	95,489	81,341	196,201	102,891	107,288	86,105	159,782	489,308	456,065	403,399
Net loss	$ (1,720)	$ (26,988)	$ (32,592)	$ 45,697	$ (14,906)	$ (14,539)	$ (19,790)	$ 35,017	$ (15,604)	$ (14,217)	$ (38,214)	$ (68,777)
Net income (loss) per share - basic (in dollars per share)	$ (0.02)	$ (0.45)	$ (0.55)	$ 0.42	$ (0.26)	$ (0.26)	$ (0.35)	$ 0.32
Net income (loss) per share - diluted (in dollars per share)	$ (0.02)	$ (0.45)	$ (0.55)	$ 0.36	$ (0.26)	$ (0.26)	$ (0.35)	$ 0.26