Offsetting of Financial Assets (Liabilities) and Derivative Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Net Assets (Liabilities) Presented in the Statement of Financial Position		$ 3,987
Net Assets (Liabilities) Presented in the Statement of Financial Position	$ (4,760)	(2,285)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	9,474	13,061
Gross Liabilities Offset in the Statement of Financial Position	(14,370)	(10,555)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(4,896)	2,506
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(4,896)	2,506
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	8,378	3,976
Gross Liabilities Offset in the Statement of Financial Position	(13,138)	(6,261)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(4,760)	(2,285)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(4,760)	(2,285)
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	922	481
Gross Liabilities Offset in the Statement of Financial Position	(1,067)	(484)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(145)	(3)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(145)	(3)
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		7,716
Gross Liabilities Offset in the Statement of Financial Position		(3,729)
Net Assets (Liabilities) Presented in the Statement of Financial Position		3,987
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount		3,987
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	174	888
Gross Liabilities Offset in the Statement of Financial Position	(165)	(81)
Net Assets (Liabilities) Presented in the Statement of Financial Position	9	807
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ 9	$ 807