Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 294,160	$ 260,119	$ 210,173	$ 496,371	$ 272,940	$ 208,398	$ 186,720	$ 468,950	$ 1,260,823	$ 1,137,008	$ 992,526
Gross profit	124,271	117,370	90,413	195,289	116,277	95,489	81,341	196,201	527,343	489,308	456,065
Net income (loss)	$ (29,600)	$ (14,009)	$ (22,824)	$ 61,667	$ (1,720)	$ (26,988)	$ (32,592)	$ 45,697	$ (4,766)	$ (15,604)	$ (14,217)
Net income (loss) per share - basic (in dollars per share)	$ (0.28)	$ (0.13)	$ (0.22)	$ 0.62	$ (0.02)	$ (0.45)	$ (0.55)	$ 0.42
Net income (loss) per share - diluted (in dollars per share)	$ (0.28)	$ (0.13)	$ (0.22)	$ 0.55	$ (0.02)	$ (0.45)	$ (0.55)	$ 0.36