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Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Effect of Fair Value Hedges on Results of Operations [Abstract]      
Commodity contracts $ (9,567) $ 3,516 $ 10,311
Fair value adjustment for natural gas inventory designated as the hedged item 12,858 18,079 (9,768)
Total (increase) decrease in purchased gas cost 3,291 21,595 543
Fair Value Hedge Basis Ineffectiveness (597) (1,390) 811
Fair Value Hedge Timing Ineffectiveness 3,888 22,985 (268)
Effect of Cash Flow Hedges on Results of Operations [Abstract]      
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts (2,612) (52,651) (41,716)
Gain (loss) arising from ineffective portion of commodity contracts 111 (19) (325)
Gain (Loss) on discontinuance of cash flow hedging of natural gas marketing commodity contracts reclassified from AOCI 10,579 0 0
Total impact on purchased gas cost 8,078 (52,670) (42,041)
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense (1,043) (546) (853)
Total Impact from Cash Flow Hedges $ 7,035 $ (53,216) $ (42,894)