Net loss per share attributable to common stockholders - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Numerator:
Net income (loss)	$ (26,988)	$ (14,539)	$ (13,884)	$ 688
Less: earnings attributable to preferred stockholders	0	0	0	(688)
Net loss attributable to common stockholders—basic	(26,988)	(14,539)	(13,884)	0
Net loss attributable to common stockholders—diluted	$ (26,988)	$ (14,539)	$ (13,884)	$ 0
Denominator:
Weighted-average shares of common stock—basic and diluted (in shares)	60,074,763	56,334,641	59,484,761	55,776,325
Net loss per share attributable to common stockholders:
Basic and diluted (in USD per share)	$ (0.45)	$ (0.26)	$ (0.23)	$ 0.00