Offsetting of Financial Assets (Liabilities) and Derivative Assets (liabilities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Net Assets (Liabilities) Presented in the Statement of Financial Position	$ 1	$ 935
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,989)	(12,819)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	23,788	31,603
Gross Liabilities Offset in the Statement of Financial Position	(30,959)	(44,489)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,171)	(12,886)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(7,171)	(12,886)
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	342	935
Gross Liabilities Offset in the Statement of Financial Position	(341)
Net Assets (Liabilities) Presented in the Statement of Financial Position	1	935
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	1	935
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	2,706
Gross Liabilities Offset in the Statement of Financial Position	(1,644)
Net Assets (Liabilities) Presented in the Statement of Financial Position	1,062
Financial Instruments	0
Cash Collateral Received	0
Net Amount	1,062
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	19,838	25,693
Gross Liabilities Offset in the Statement of Financial Position	(27,827)	(38,512)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(7,989)	(12,819)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(7,989)	(12,819)
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	902	4,975
Gross Liabilities Offset in the Statement of Financial Position	(1,147)	(5,977)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(245)	(1,002)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	$ (245)	$ (1,002)