Pensions and Other Postretirement Benefits - Summary of Pension and Other Postretirement Benefit Plans (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Aug. 31, 2012
Aug. 31, 2011
Aug. 31, 2010
Change in plan assets:      
Actual return on plan assets       
Company contributions 4,100 6,400 3,400
Balance, August 31, 2012 42,072    
Pension Benefits [Member]
      
Change in benefit obligation:      
Benefit obligation, beginning balance 47,545 48,875  
Service cost 1,521 1,617 1,608
Interest cost 2,729 2,696 2,633
Amendments 212    
Actuarial (gain) loss 619 (1,099)  
Change in assumptions 12,359 (2,560)  
Benefits paid (1,996) (1,984)  
Benefit obligation, ending balance 62,989 47,545 48,875
Change in plan assets:      
Balance, August 31, 2011 36,328 28,278  
Actual return on plan assets 3,688 3,618  
Company contributions 4,052 6,416  
Benefits paid (1,996) (1,984)  
Balance, August 31, 2012 42,072 36,328 28,278
Funded status:      
Net liability - Plan assets less than projected benefit obligation (20,917) (11,217)  
Recognized as:      
Non-current accrued benefit liability (20,917) (11,217)  
Net Amount Recognized (20,917) (11,217)  
Other Benefits [Member]
      
Change in benefit obligation:      
Benefit obligation, beginning balance 16,924 17,580  
Service cost 229 272 353
Interest cost 972 972 1,077
Plan participants' contributions 119 134  
Actuarial (gain) loss (170) 333  
Change in assumptions 3,024 (1,736)  
Benefits paid (641) (631)  
Benefit obligation, ending balance 20,457 16,924 17,580
Change in plan assets:      
Company contributions 522 497  
Plan participants' contributions 119 134  
Benefits paid (641) (631)  
Funded status:      
Net liability - Plan assets less than projected benefit obligation (20,457) (16,924)  
Recognized as:      
Current accrued benefit liability (750) (731)  
Non-current accrued benefit liability (19,707) (16,193)  
Net Amount Recognized $ (20,457) $ (16,924)