Earnings (Loss) Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Oct. 28, 2017	Oct. 29, 2016
Numerator:
Net income	$ 13,488	$ 13,243
Less: noncumulative dividends to preferred stockholders	(635)	(635)
Less: undistributed earnings to participating securities	(8,938)	(9,013)
Net income attributable to common stockholders - basic	3,915	3,595
Less: change in fair value of preferred stock warrant liability (net of tax)	(9,071)
Add: adjustments to undistributed earnings to participating securities	6,503	460
Net income attributable to common stockholders - diluted	$ 1,347	$ 4,055
Shares used to compute earnings per share attributable to common stockholders:
Basic	26,329,495	24,349,434
Weighted-average shares of common stock - diluted	33,262,082	28,940,058
Net income attributable to common stockholders:
Basic	$ 0.15	$ 0.15
Diluted	$ 0.04	$ 0.14
Stock Options
Shares used to compute earnings per share attributable to common stockholders:
Effect of dilutive stock options and preferred stock warrants	5,876,621	4,590,624
Preferred Stock Warrants
Shares used to compute earnings per share attributable to common stockholders:
Effect of dilutive stock options and preferred stock warrants	1,055,966