Net Earnings (Loss) Per Common Unitholders' Interest (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Common unitholders' interest in net earnings (loss)	$ 55,069	$ 36,010	$ 40,144	$ 3,732
Weighted average common units outstanding	79,015,600	76,401,600	79,014,400	76,184,000
Dilutive securities	36,100	62,200	40,600	65,100
Weighted average common units outstanding plus dilutive securities	79,051,700	76,463,800	79,055,000	76,249,100
Basic and diluted net earnings per common unitholders' interest	$ 0.70	$ 0.47	$ 0.51	$ 0.05