Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ 5,744	$ 15,090
Fair value adjustment for natural gas inventory designated as the hedged item	2,161	(16,782)
Total (increase) decrease to purchased gas cost	7,905	(1,692)
Basis ineffectiveness	(1,289)	(986)
Timing ineffectiveness	(6,616)	2,678
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(22,965)	344
Gain (loss) arising from ineffective portion of commodity contracts	(43)	(490)
Total impact on purchased gas cost	(23,008)	(146)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(137)	(444)
Total Impact from Cash Flow Hedges	(23,145)	(590)
Regulated Distribution Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	0	0
Gain (loss) arising from ineffective portion of commodity contracts	0	0
Total impact on purchased gas cost	0	0
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(137)	(444)
Total Impact from Cash Flow Hedges	(137)	(444)
Nonregulated Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(22,965)	344
Gain (loss) arising from ineffective portion of commodity contracts	(43)	(490)
Total impact on purchased gas cost	(23,008)	(146)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	0	0
Total Impact from Cash Flow Hedges	$ (23,008)	$ (146)