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, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	$ 35,659	$ 26,263
Derivative Liabilities, commodity contracts	(42,710)	(36,279)
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	34,566	26,263
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	(41,647)	(36,279)
Long-term derivative assets included in the deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	1,093
Long-term derivative liabilities included in other long-term liabilities balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	(1,063)
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	8,304	2,328
Derivative Liabilities, commodity contracts	(7,713)
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	8,124	2,328
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	(7,591)
Quoted Prices in Active Markets for Identical Assets Level 1 | Long-term derivative assets included in the deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	180
Quoted Prices in Active Markets for Identical Assets Level 1 | Long-term derivative liabilities included in other long-term liabilities balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	(122)
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	27,355	23,935
Derivative Liabilities, commodity contracts	(34,997)	(36,279)
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	26,442	23,935
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	(34,056)	(36,279)
Significant Other Observable Inputs Level 2 | Long-term derivative assets included in the deferred charges and other assets, net balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, commodity contracts	913
Significant Other Observable Inputs Level 2 | Long-term derivative liabilities included in other long-term liabilities balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, commodity contracts	$ (941)