Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ (9,567)	$ 5,744
Fair value adjustment for natural gas inventory designated as the hedged item	12,858	2,161
Total (increase) decrease to purchased gas cost	3,291	7,905
Basis ineffectiveness	(597)	1,289
Timing ineffectiveness	3,888	6,616
Summary of Cash Flow Hedge Activity [Abstract]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(2,612)	(22,965)
Gain (loss) arising from ineffective portion of commodity contracts	111	(43)
Gain (Loss) on Discontinuation of Cash Flow Hedge	10,579	0
Total impact on purchased gas cost	8,078	(23,008)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(137)	(137)
Total Impact from Cash Flow Hedges	$ 7,941	$ (23,145)