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Goodwill and Intangible Assets - Amortizable intangible assets by major asset class (Details) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2013
Jul. 31, 2012
Finite-Lived Intangible Assets [Line Items]    
Accumulated Amortization $ (34,234) $ (30,459)
Net intangibles 13,100 7,985
Covenants not to compete
   
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 11,430 11,087
Supply contracts
   
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 31,650 26,041
Trade name
   
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 2,440   
Licenses and databases
   
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount 1,552 1,316
Software
   
Finite-Lived Intangible Assets [Line Items]    
Gross Carrying Amount $ 262