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Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Effect of Fair Value Hedges on Results of Operations [Abstract]        
Commodity contracts $ 0 $ 4,594 $ (9,567) $ 10,338
Fair value adjustment for natural gas inventory designated as the hedged item 0 (7,939) 12,858 (5,778)
Total (increase) decrease to purchased gas cost 0 (3,345) 3,291 4,560
Basis ineffectiveness 0 (2,095) 597 806
Timing ineffectiveness 0 (1,250) (3,888) (5,366)
Summary of Cash Flow Hedge Activity [Abstract]        
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts 0 (12,703) (2,612) (35,668)
Gain (loss) arising from ineffective portion of commodity contracts 0 61 111 18
Gain (Loss) on Discontinuation of Cash Flow Hedge 0 0 10,579 0
Total impact on purchased gas cost 0 (12,642) 8,078 (35,650)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense (136) (136) (273) (273)
Total Impact from Cash Flow Hedges $ (136) $ (12,778) $ 7,805 $ (35,923)