Lease Commitments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Railcar Equipment
Lessee, Lease, Description [Line Items]
Lease expense	$ 7.6	$ 6.6	$ 6.3
Domestic railcar repair facilities, office space and certain manufacturing and office equipment
Lessee, Lease, Description [Line Items]
Lease expense	$ 9.4	$ 9.3	$ 9.3

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

No definition available.

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration