Offsetting of Financial Assets (Liabilities) and Derivative Assets (liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Net Assets (Liabilities) Presented in the Statement of Financial Position	$ 569us-gaap_DerivativeAssetsCurrent	$ 2,342us-gaap_DerivativeAssetsCurrent
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,650)us-gaap_DerivativeLiabilitiesCurrent	(12,358)us-gaap_DerivativeLiabilitiesCurrent
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	35,659	26,263
Gross Liabilities Offset in the Statement of Financial Position	(42,710)	(36,279)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,051)	(10,016)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(7,051)	(10,016)
Subject to an enforceable master netting arrangement \| Fair liability and fair asset value of derivative instruments
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	26,407	23,921
Gross Liabilities Offset in the Statement of Financial Position	(34,057)	(36,279)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,650)	(12,358)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(7,650)	(12,358)
Subject to an enforceable master netting arrangement \| Fair asset and fair liability value of derivative instruments
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	8,159	2,342
Gross Liabilities Offset in the Statement of Financial Position	(7,590)
Net Assets (Liabilities) Presented in the Statement of Financial Position	569	2,342
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	569	2,342
Subject to an enforceable master netting arrangement \| Long-term derivative liabilities included in other long-term liabilities balance
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	421
Gross Liabilities Offset in the Statement of Financial Position	(513)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(92)
Financial Instruments	0
Cash Collateral Received	0
Net Amount	(92)
Subject to an enforceable master netting arrangement \| Long-term derivative assets included in the deferred charges and other assets, net balance
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	672
Gross Liabilities Offset in the Statement of Financial Position	(550)
Net Assets (Liabilities) Presented in the Statement of Financial Position	122
Financial Instruments	0
Cash Collateral Received	0
Net Amount	$ 122

Cash Collateral Received From Not Offset Against Derivative Assets and Liabilities

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Derivative Fair Value Gross Amount Not Offset Financial Instruments Net

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Amount of obligation to return securities collateral under master netting arrangements that have not been offset against derivative assets.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Amount of right to receive securities collateral under master netting arrangements that have not been offset against derivative liabilities.

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after effects of master netting arrangements, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against an obligation to return collateral. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after effects of master netting arrangements, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset offset against the right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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