Contingencies and Legacy Environmental Commitments - Schedule of Environmental Activity (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013
Environmental Remediation Obligations [Abstract]
Environmental activity, Beginning balance	$ 12,684
Environmental costs incurred	(602)	(544)	(1,245)
Interest accretion	174
Reduction of environmental reserve-Wyoming Illinois	(1,207)
Environmental activity, Ending balance	$ 11,049		$ 12,684

Aggregate environmental liability.

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- Definition

Environmental Liability Additional Accrual Interest Accretion

+ References

No definition available.

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- Definition

The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859

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- Definition

Other increase (decrease) not attributable to current expense nor cash payments during an accounting period in estimated obligations recorded for probable future costs attributable to environmental contamination issues.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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