Offsetting of Financial Assets (Liabilities) and Derivative Assets (liabilities) (Detail) - USD ($) $ in Thousands	Sep. 30, 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	$ 3,987	$ 935
Net Assets (Liabilities) Presented in the Statement of Financial Position	(2,285)	(12,819)
Net Amount	(40)	(500)
Subject to an enforceable master netting arrangement
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]
Gross Assets Recognized	13,061	31,603
Gross Liabilities Offset in the Statement of Financial Position	(10,555)	(44,489)
Net Assets (Liabilities) Presented in the Statement of Financial Position	2,506	(12,886)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	2,506	(12,886)
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	3,976	25,693
Gross Liabilities Offset in the Statement of Financial Position	(6,261)	(38,512)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(2,285)	(12,819)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(2,285)	(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	481	4,975
Gross Liabilities Offset in the Statement of Financial Position	(484)	(5,977)
Net Assets (Liabilities) Presented in the Statement of Financial Position	(3)	(1,002)
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	(3)	(1,002)
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	935
Gross Liabilities Offset in the Statement of Financial Position	(3,729)
Net Assets (Liabilities) Presented in the Statement of Financial Position	3,987	935
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Amount	3,987	$ 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	888
Gross Liabilities Offset in the Statement of Financial Position	(81)
Net Assets (Liabilities) Presented in the Statement of Financial Position	807
Financial Instruments	0
Cash Collateral Received	0
Net Amount	$ 807