Commitments and Contingencies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended	24 Months Ended
	Jun. 30, 2011	Sep. 30, 2009	Mar. 31, 2013 gal	Sep. 30, 2012	Sep. 30, 2006
Commitment And Contingencies [Line Items]
Environmental Remediation Charges	$ 6,000	$ 13,700		$ 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			13,847	16,754
Minimum
Commitment And Contingencies [Line Items]
Environmental Remediation Charges			10,000
Maximum
Commitment And Contingencies [Line Items]
Environmental Remediation Charges			$ 33,900