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Benefit Plans - Narative (Details) (USD $)
12 Months Ended
Aug. 31, 2014
Aug. 31, 2013
Aug. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, health care benefits annual rate of increase in the per capita cost 6.90%    
Defined benefit plan, health care benefits annual rate of increase in the per capita cost trend rate 4.80%    
Defined contribution plan, contributions by employer $ 24,600,000 $ 22,900,000 $ 20,600,000
Pension protection act, percentage of employers that are rural cooperatives or cooperative organizations owned by agricultural producers, criteria 85.00%    
Fixed Income Securities
     
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, target allocation percentage 50.00%    
Equity Securities
     
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, target allocation percentage 50.00%    
Qualified Pension Benefits
     
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation 682,100,000 605,600,000  
Company contributions 25,700,000 23,800,000  
Defined benefit plan, expected future benefit payments, next twelve months 35,797,000    
Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
     
Defined Benefit Plan Disclosure [Line Items]      
Number of employers contributing to Co-op Retirement Plan (in number of employers) 400    
Co-op Retirement Plan - Contributions of CHS 2,079,000 2,095,000 1,885,000
Pension Plans And Postretirement Plans, Defined Benefit, Non-Qualified
     
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, expected future benefit payments, next twelve months 6,000,000    
Other Benefits
     
Defined Benefit Plan Disclosure [Line Items]      
Company contributions 2,161,000 2,014,000  
Non-Qualified Pension Benefits
     
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation 22,700,000 20,100,000  
Company contributions 3,576,000 1,785,000  
Defined benefit plan, expected future benefit payments, next twelve months $ 3,222,000    
Maximum | Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
     
Defined Benefit Plan Disclosure [Line Items]      
Employer contributions as percent of total contributions of all contributing employers 5.00%    
Minimum | Qualified Pension Benefits | Multiemployer Plans, Pension | Co-op Retirement Plan
     
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, funded percentage 80.00%