Environmental and Other Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011 Bushel Project Location
Commitments and Contingencies Disclosure [Line Items]
Superfund site the company is considered to be a Potentially Responsible Party (PRP)	1
Financial Guarantee
Commitments and Contingencies Disclosure [Line Items]
Contingent liabilities	$ 7.8
Letter of Credit
Commitments and Contingencies Disclosure [Line Items]
Contingent liabilities	$ 251.5

The estimated maximum amount of possible loss from the contingency.

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