Offsetting of Financial Assets (Liabilities) and Derivative Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
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	$ 5,932
Net Assets (Liabilities) Presented in the Statement of Financial Position		(289)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	18,616	8,725
Gross Liabilities Offset in the Statement of Financial Position	(103)	(2,817)
Net Assets (Liabilities) Presented in the Statement of Financial Position	18,513	5,908
Financial Instruments		0
Cash Collateral Received		0
Net Amount	18,513	5,908
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	6,023
Gross Liabilities Offset in the Statement of Financial Position		(91)
Net Assets (Liabilities) Presented in the Statement of Financial Position	17,710	5,932
Financial Instruments		0
Cash Collateral Received		0
Net Amount	17,710	5,932
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	996
Gross Liabilities Offset in the Statement of Financial Position	(103)	(730)
Net Assets (Liabilities) Presented in the Statement of Financial Position	803	266
Financial Instruments		0
Cash Collateral Received		0
Net Amount	$ 803	266
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		1,706
Gross Liabilities Offset in the Statement of Financial Position		(1,995)
Net Assets (Liabilities) Presented in the Statement of Financial Position		(289)
Financial Instruments		0
Cash Collateral Received		0
Net Amount		(289)
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 Liabilities Offset in the Statement of Financial Position		(1)
Net Assets (Liabilities) Presented in the Statement of Financial Position		(1)
Financial Instruments		0
Cash Collateral Received		0
Net Amount		$ (1)