Partnership's Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - Derivatives Not Designated as Hedging Instruments under FASB ASC 815-10 - Commodity Contract - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 23,788	$ 31,603
Derivative Liabilities, commodity contracts	(30,959)	(44,489)
Fair asset and fair liability value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	20,180	26,628
Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	(28,168)	(41,270)
Cash Collateral
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts		2,758
Other long-term liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts		4,975
Derivative Liabilities, commodity contracts		(5,977)
Deferred Charges and Other Assets, Net, and Other Long-Term Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	3,608
Derivative Liabilities, commodity contracts	(2,791)
Quoted Prices in Active Markets for Identical Assets Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	4,171	2,947
Derivative Liabilities, commodity contracts	(5,811)	720
Quoted Prices in Active Markets for Identical Assets Level 1 | Fair asset and fair liability value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	2,817	930
Quoted Prices in Active Markets for Identical Assets Level 1 | Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	(5,176)
Quoted Prices in Active Markets for Identical Assets Level 1 | Cash Collateral
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts		2,758
Quoted Prices in Active Markets for Identical Assets Level 1 | Other long-term liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts		2,017
Derivative Liabilities, commodity contracts		(2,038)
Quoted Prices in Active Markets for Identical Assets Level 1 | Deferred Charges and Other Assets, Net, and Other Long-Term Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	1,354
Derivative Liabilities, commodity contracts	(635)
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	19,617	28,656
Derivative Liabilities, commodity contracts	(25,148)	(45,209)
Significant Other Observable Inputs Level 2 | Fair asset and fair liability value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	17,363	25,698
Significant Other Observable Inputs Level 2 | Fair liability and fair asset value of derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	(22,992)	(41,270)
Significant Other Observable Inputs Level 2 | Other long-term liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts		2,958
Derivative Liabilities, commodity contracts		$ (3,939)
Significant Other Observable Inputs Level 2 | Deferred Charges and Other Assets, Net, and Other Long-Term Liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	2,254
Derivative Liabilities, commodity contracts	$ (2,156)