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)