Derivatives and Hedging-Disclosures and Fair Value Measurements - Offsetting of Financial Assets (Liabilities) and Derivative Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Net Assets (Liabilities) Presented in the Statement of Financial Position		$ 17,710
Net Assets (Liabilities) Presented in the Statement of Financial Position	$ (2,418)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	11,368	18,616
Gross Liabilities Offset in the Statement of Financial Position	(13,823)	(103)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(2,455)	18,513
Financial Instruments		0
Cash Collateral Received		0
Net Amount	(2,455)	18,513
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		17,710
Net Assets (Liabilities) Presented in the Statement of Financial Position		17,710
Financial Instruments		0
Cash Collateral Received		0
Net Amount		17,710
Subject to an enforceable master netting arrangement \| Deferred charges and other assets, net
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	112
Gross Liabilities Offset in the Statement of Financial Position	(92)
Net Assets (Liabilities) Presented in the Statement of Financial Position	20
Net Amount	20
Subject to an enforceable master netting arrangement \| Other long-term assets, net
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized		906
Gross Liabilities Offset in the Statement of Financial Position		(103)
Net Assets (Liabilities) Presented in the Statement of Financial Position		803
Financial Instruments		0
Cash Collateral Received		0
Net Amount		803
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	10,299
Gross Liabilities Offset in the Statement of Financial Position	(12,717)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(2,418)
Financial Instruments		0
Cash Collateral Received		0
Net Amount	(2,418)
Subject to an enforceable master netting arrangement \| Other long-term liabilities
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	957
Gross Liabilities Offset in the Statement of Financial Position	(1,014)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(57)
Financial Instruments		0
Cash Collateral Received		$ 0
Net Amount	$ (57)

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