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Financial Instruments and Risk Management - Fair Value of Partnership's Recognized Derivative Assets and Liabilities on a Gross Basis and Amounts Offset on Condensed Consolidated Balance Sheets (Details) - Commodity-Related Derivatives [Member] - USD ($)
$ in Thousands
Jun. 30, 2018
Sep. 30, 2017
Asset Derivatives [Abstracts]    
Gross amounts $ 19,688 $ 16,378
Effects of netting (8,577) (4,443)
Net amounts presented in the balance sheet 11,111 11,935
Liability Derivatives [Abstracts]    
Gross amounts 13,539 6,853
Effects of netting (8,577) (4,443)
Net amounts presented in the balance sheet $ 4,962 $ 2,410