Commitments and Contingencies -Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended	12 Months Ended				24 Months Ended
	Jun. 30, 2011	Jun. 30, 2012 gal	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009	Sep. 30, 2006
Site Contingency [Line Items]
Total rental expense under operating leases			$ 1,735	$ 1,642	$ 1,587
Operating lease expiration year			2018
Estimated minimum aggregate severance benefits			4,700
Period of ground water investigation, minimum			6 years
Environmental Remediation Charges	6,000		700	6,000		13,700	26,000
Extension of remediation project, minimum			50 years
Estimated increase in capital cost of expansion project	6,400
Estimate cost of O&M	400
Measurement for new ground water extraction wells		450
Measurement for existing wells		750
Estimated life of project, minimum		5 years
Estimated life of project, maximum		18 years
More likely estimated life of project, minimum		7 years
More likely estimated life of project, maximum		14 years
Maturity life of the project		2019
Estimated project costs	1,900
Amount accrued through estimated life of the project			16,754	26,173
Maximum
Site Contingency [Line Items]
Environmental Remediation Charges			36,900
Minimum
Site Contingency [Line Items]
Environmental Remediation Charges			$ 13,000