Offsetting of Financial Assets (Liabilities) and Derivative Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	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	$ (18,065)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	40,440	18,616
Gross Liabilities Offset in the Statement of Financial Position	(58,757)	(103)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(18,317)	18,513
Financial Instruments		0
Cash Collateral Received		0
Net Amount	(18,317)	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	17
Gross Liabilities Offset in the Statement of Financial Position	(14)
Net Assets (Liabilities) Presented in the Statement of Financial Position	3
Net Amount	3
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	38,247
Gross Liabilities Offset in the Statement of Financial Position	(56,312)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(18,065)
Financial Instruments		0
Cash Collateral Received		0
Net Amount	(18,065)
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	2,176
Gross Liabilities Offset in the Statement of Financial Position	(2,431)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(255)
Financial Instruments		0
Cash Collateral Received		$ 0
Net Amount	$ (255)

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