Net income (loss) per share attributable to common stockholders - Computation of Basic and Diluted Earnings per Share (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Aug. 06, 2018 shares	Mar. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common shares issued in conversion of outstanding shares of redeemable convertible preferred stock (shares) | shares	32,482,590
Number of shares of common stock issued for each share of redeemable convertible preferred stock (shares)	1
Numerator:
Net income		$ (22,824)	$ (32,592)	$ 38,843	$ 13,105
Less: noncumulative dividend to preferred stockholders		0	0	0	(7,293)
Less: undistributed earnings to participating securities		0	0	0	(2,059)
Net loss attributable to common stockholders—basic		(22,824)	(32,592)	38,843	3,753
Add: undistributed earnings reallocated to holders of common stock		0	0	0	275
Net loss attributable to common stockholders—diluted		$ (22,824)	$ (32,592)	$ 38,843	$ 4,028
Denominator:
Weighted-average shares of common stock - basic (in shares) | shares		102,331,529	59,606,879	101,239,817	59,189,760
Weighted-average shares of common stock - diluted (in shares) | shares		102,331,529	59,606,879	111,474,057	73,365,673
Net loss per share attributable to common stockholders:
Net income per share attributable to common stockholders - basic (in USD per share) | $ / shares		$ (0.22)	$ (0.55)	$ 0.38	$ 0.06
Net income per share attributable to common stockholders - diluted (in USD per share) | $ / shares		$ (0.22)	$ (0.55)	$ 0.35	$ 0.05
Stock options
Denominator:
Effect of potentially dilutive stock options and RSUs | shares		0	0	9,901,628	14,175,913
Restricted Stock Units
Denominator:
Effect of potentially dilutive stock options and RSUs | shares		0	0	332,612	0