Earnings Per Share (Details) (USD $)	3 Months Ended		6 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Net income for basic calculation	$ 2,365,000	$ 5,821,000	$ 9,800,000	$ 18,422,000
Interest expense (net of tax) on 3.0% convertible senior notes	0	1,117,000	2,234,000	2,234,000
Numerator for diluted calculation	2,365,000	6,938,000	12,034,000	20,656,000
Denominator for basic calculation (in shares)	17,300,000	20,087,000	17,340,000	21,672,000
Dilutive effect on shares of stock-based awards (in shares)	101,000	228,000	105,000	200,000
Dilutive effect on shares of conversion of 3.0% convertible senior notes (in shares)	0	5,741,000	5,949,000	5,716,000
Denominator for diluted calculation (in shares)	17,401,000	26,056,000	23,394,000	27,588,000
Stock-Based Awards [Member]
Anti-dilutive shares excluded from computation of diluted earnings per share (in shares)	2,658,000	2,147,000	2,680,000	2,647,000
3.0% Convertible Senior Notes [Member]
Anti-dilutive shares excluded from computation of diluted earnings per share (in shares)	5,992,000
Anti-dilutive interest expense (net of tax) excluded from computation of diluted earnings per share	$ 1,117,000
Treasury Stock [Member]
Reduction in weighted average shares as a result of the repurchase of common shares	102,000	1,544,000	51,000	2,627,000