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Derivative Instruments and Hedging Activities - Effects of Derivatives on Statements of Income and AOCI (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Derivatives Not Subject to PGC and DS Mechanisms: | Operating and administrative expenses        
Derivative Instruments, Gain (Loss)        
Location of (Loss) Gain Recognized in Income $ 10 $ 37    
Not designated as hedging instruments | Commodity contracts | Operating and administrative expenses        
Derivative Instruments, Gain (Loss)        
Location of (Loss) Gain Recognized in Income     $ (257) $ 198
Cash Flow Hedges | Interest rate contracts        
Derivative Instruments, Gain (Loss)        
Loss Recognized in AOCI (1,683) 0 (4,241) 0
Cash Flow Hedges | Interest rate contracts | Interest expense        
Derivative Instruments, Gain (Loss)        
Location of Loss Reclassified from AOCI into Income $ (872) $ (871) $ (2,614) $ (2,614)