Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ 4,594	$ (7,622)	$ 10,338	$ 7,469
Fair value adjustment for natural gas inventory designated as the hedged item	(7,939)	5,142	(5,778)	(11,641)
Total (increase) decrease to purchased gas cost	(3,345)	(2,480)	4,560	(4,172)
Basis ineffectiveness	(2,095)	(678)	(806)	309
Timing ineffectiveness	(1,250)	(1,802)	5,366	(4,481)
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(12,703)	(13,078)	(35,668)	(12,734)
Gain (loss) arising from ineffective portion of commodity contracts	61	163	18	(327)
Total impact on purchased gas cost	(12,642)	(12,915)	(35,650)	(13,061)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(136)	(136)	(273)	(580)
Total Impact from Cash Flow Hedges	(12,778)	(13,051)	(35,923)	(13,641)
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	0	0
Gain (loss) arising from ineffective portion of commodity contracts	0	0	0	0
Total impact on purchased gas cost	0	0	0	0
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(136)	(136)	(273)	(580)
Total Impact from Cash Flow Hedges	(136)	(136)	(273)	(580)
Nonregulated Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(12,703)	(13,078)	(35,668)	(12,734)
Gain (loss) arising from ineffective portion of commodity contracts	61	163	18	(327)
Total impact on purchased gas cost	(12,642)	(12,915)	(35,650)	(13,061)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	0	0	0	0
Total Impact from Cash Flow Hedges	$ (12,642)	$ (12,915)	$ (35,650)	$ (13,061)