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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
Mar. 31, 2018
Sep. 30, 2017
Asset Derivatives [Abstracts]    
Gross amounts $ 8,912 $ 16,378
Effects of netting (3,293) (4,443)
Net amounts presented in the balance sheet 5,619 11,935
Liability Derivatives [Abstracts]    
Gross amounts 5,116 6,853
Effects of netting (3,293) (4,443)
Net amounts presented in the balance sheet $ 1,823 $ 2,410