Commitments and Contingencies - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	May. 31, 2015	Aug. 31, 2014	Jul. 18, 2014
Commitments and Contingencies Disclosure [Line Items]
Remedial investigation and feasibility study	$ 110,000,000
Remedial investigation and feasibility study, period	14 years
Remedial investigation and feasibility study, expiration year	2017
Bank and third party warranty and performance guarantee facilities	$ 3,500,000
GBW Railcar Services Holdings, LLC
Commitments and Contingencies Disclosure [Line Items]
Equity method investment, percentage of ownership interest			50.00%
Payment to acquire equity method investments	3,200,000	$ 12,500,000
Account receivable	21,000,000
Note receivable	7,500,000
Proceeds from lease receivables	5,000,000
Quarterly distributions received	700,000
GBW Railcar Services Holdings, LLC | Watco Companies LLC
Commitments and Contingencies Disclosure [Line Items]
Equity method investment, percentage of ownership interest			50.00%
Performance Guarantee
Commitments and Contingencies Disclosure [Line Items]
Outstanding letters of credit	49,800,000
Mexican Joint Venture Credit Facility
Commitments and Contingencies Disclosure [Line Items]
Debt outstanding	20,800,000
Guarantee obligations	$ 18,800,000
Minimum
Commitments and Contingencies Disclosure [Line Items]
Period for remedial action	2 years
Maximum
Commitments and Contingencies Disclosure [Line Items]
Period for remedial action	28 years
Maximum | GBW Railcar Services Holdings, LLC
Commitments and Contingencies Disclosure [Line Items]
Expected loan to unconsolidated joint venture	$ 7,000,000
Portland Harbor Site | Minimum
Commitments and Contingencies Disclosure [Line Items]
Cleanup expenses	169,000,000
Portland Harbor Site | Maximum
Commitments and Contingencies Disclosure [Line Items]
Cleanup expenses	1,800,000,000
Area Of Willamette River Adjacent to the Company's Portland, Oregon Manufacturing Facility | Minimum
Commitments and Contingencies Disclosure [Line Items]
Cleanup expenses	9,000,000
Area Of Willamette River Adjacent to the Company's Portland, Oregon Manufacturing Facility | Maximum
Commitments and Contingencies Disclosure [Line Items]
Cleanup expenses	$ 163,000,000