Derivatives and Hedging-Disclosures and Fair Value Measurements - Offsetting of Financial Assets (Liabilities) and Derivative Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Net Assets (Liabilities) Presented in the Statement of Financial Position	$ (14,017)	$ (8,262)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	37,170	15,290
Gross Liabilities Offset in the Statement of Financial Position	(51,381)	(23,805)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(14,211)	(8,515)
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments		0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received		0
Gross Amounts Not Offset in the Statement of Financial Position, Net Amount	(14,211)	(8,515)
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	615
Gross Liabilities Offset in the Statement of Financial Position	(559)
Net Assets (Liabilities) Presented in the Statement of Financial Position	56
Gross Amounts Not Offset in the Statement of Financial Position, Net Amount	56
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		16
Gross Liabilities Offset in the Statement of Financial Position		(16)
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments		0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received		0
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	35,497	13,824
Gross Liabilities Offset in the Statement of Financial Position	(49,514)	(22,086)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(14,017)	(8,262)
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments		0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received		0
Gross Amounts Not Offset in the Statement of Financial Position, Net Amount	(14,017)	(8,262)
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	1,058	1,450
Gross Liabilities Offset in the Statement of Financial Position	(1,308)	(1,703)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(250)	(253)
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments		0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received		0
Gross Amounts Not Offset in the Statement of Financial Position, Net Amount	$ (250)	$ (253)

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