Computation of Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jan. 28, 2017	Jan. 23, 2016	Jan. 28, 2017	Jan. 23, 2016	Sep. 15, 2015
Basic earnings per unit
Net income	$ 23,663	$ 15,473	$ 74,713	$ 46,297
Weighted-average number of common shares (in shares)	31,531,834	32,662,942	31,480,660	32,767,088
Basic earnings per common share (in dollars per share)	$ 0.75	$ 0.47	$ 2.37	$ 1.41
Diluted earnings per unit
Weighted-average number of common shares (in shares)	31,531,834	32,662,942	31,480,660	32,767,088
Potential common stock arising from stock options, and unvested restricted share units (in shares)	629,732	857,194	700,263	936,350
Total shares-diluted (in shares)	32,161,566	33,520,136	32,180,923	33,703,438
Diluted earnings per common share (in dollars per share)	$ 0.74	$ 0.46	$ 2.32	$ 1.37
Anti-dilutive weighted shares excluded from the calculation of earnings per share (in shares)	10,070,218	10,074,825	10,061,823	10,072,949
Stock-based awards
Diluted earnings per unit
Anti-dilutive weighted shares excluded from the calculation of earnings per share (in shares)	58,750	63,357	50,355	61,481
Convertible senior notes
Diluted earnings per unit
Anti-dilutive weighted shares excluded from the calculation of earnings per share (in shares)	5,005,734	5,005,734	5,005,734	5,005,734
Warrants
Diluted earnings per unit
Anti-dilutive weighted shares excluded from the calculation of earnings per share (in shares)	5,005,734	5,005,734	5,005,734	5,005,734
Convertible Note Hedge
Diluted earnings per unit
Debt Instrument, Convertible, Conversion Price (per share)					$ 96.89
0.75% Convertible Senior Notes Due 2021
Diluted earnings per unit
Debt Instrument, Convertible, Conversion Price (per share)					96.89
Class of Warrant or Right, Exercise Price of Warrants or Rights					$ 130.43